Ping-pong state machine for storage-device array

ABSTRACT

In one embodiment, an apparatus for reading from a physical storage-device array including a plurality of storage devices. The physical storage-device array has a plurality of sector levels, each sector level containing one sector on each storage device in the array at corresponding addresses on across the storage devices. The apparatus includes: (1) a memory adapted to store two or more profiles, each profile defining (i) a virtual array associated with a selected set of the storage devices and (ii) one or more parameters used for accessing information from the virtual array; (2) a buffer (i) having a first portion and a second portion and (ii) coupled to receive data from the storage devices; and (3) a state machine (i) coupled to the buffer and the memory and (ii) adapted to generate two or more successive pairs of instructions. The first instruction of each pair of instructions (i) is based on one or more parameters of a first profile and (ii) is an instruction to read, into the first portion of the buffer, one sector of data from each storage device of a first virtual array; and the second instruction of each pair of instructions (i) is based on one or more parameters of a second profile and (ii) is an instruction to read, into the second portion of the buffer, one sector of data from each storage device of a second virtual array.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing date of U.S.provisional application No. 60/724,573, filed on Oct. 7, 2005, theteachings of which are incorporated herein by reference.

This application is one of a set of U.S. patent applications consistingof Ser. No. 11/544,442 filed as Ser. No. 11/544,462 filed as Ser. No.11/544,445 filed as and Ser. No. 11/544,456, all of which were filed onthe same date and the teachings of all of which are incorporated hereinby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to data storage devices, and, inparticular, to arrays of disks for storing data.

2. Description of the Related Art

In general, there are several defined categories of storage schemes thatare used in conjunction with a Redundant Array of Independent (orInexpensive) Disks (RAID). Different hardware and software componentssupplied by different vendors may support one or more of these schemes,which are identified as RAID “levels” having particular specifications,as follows.

RAID level 0 (or “RAID-0”) specifies a block-interleaved, striped diskarray without fault tolerance and requires a minimum of two drives toimplement. In a RAID-0 striped disk arran, the data broken down intoblocks, and each block is written to a separate disk drive in the array.Input/output (I/O) performance is greatly improved by spreading the I/Oload across a plurality of channels and drives. In RAID-0, optimalperformance is achieved when data is striped across multiple controllerswith only one drive per controller. RAID-0 involves no paritycalculation overhead and is not a “true” RAID because it is notfault-tolerant, i.e., there is no redundancy of data. Thus, the failureof only one drive will result in all of the data in an array being lost.FIG. 1 illustrates the sequence of storing blocks in an exemplary RAID-0striped disk array, wherein block A is written to the first disk, blockB is written to the second disk, block C is written to the third disk,block D is written to the first disk, and so forth.

RAID-1 specifies a disk array with mirroring (redundancy) of data acrossdifferent physical hard disks. In a RAID-1 array, each block of data ona disk exists in identical form on another disk in the array. Foroptimal performance, the controller performs two concurrent separatereads per mirrored disk pair and two duplicate writes per mirrored diskpair. RAID-1 requires a minimum of two drives to implement and makesdata recovery following a disk failure relatively easy. FIG. 2illustrates the sequence of storing blocks in an exemplary RAID-1mirrored disk array, wherein block A is written to the first disk, acopy A′ of block A is written to the second disk, block B is written tothe first disk, a copy B′ of block B is written to the second disk, andso forth.

RAID-4 specifies a block-interleaved, dedicated parity-disk array. InRAID-4, each entire block is written onto data disks, and a non-datadisk called a parity disk is used to store parity blocks. Each parityblock is typically generated by exclusive-OR (XOR) combining datacontained in corresponding same-rank blocks on the data disks. Toprovide write verification, RAID-4 specifies that writes to the paritydisk take place for each data block stored on a data disk. To provideread verification, reads from the parity disk take place for each datablock that is read from a data disk. RAID-4 requires a minimum of threedrives to implement and has a relatively high read-data transactionrate. High efficiency of a RAID-4 array correlates with a lowparity-disk/data-disk ratio. RAID-4 exhibits relatively high read-datatransaction rates, relatively high aggregate-read-transfer rates, andblock-read-transfer rates equal to those of a single disk.Disadvantageously, however, RAID-4 has low write-transaction rates andrelatively low write-aggregate-transfer rates. However, data can berebuilt in the event of disk failure. FIG. 3 illustrates the sequence ofstoring blocks in an exemplary RAID-4 dedicated-parity disk array,wherein block A is written to the first disk, block B is written to thesecond disk, and then a parity block is generated by XOR-combiningblocks A and B. The parity block pAB for blocks A and B is stored on thethird disk. Block C is then written to the first disk, block D iswritten to the second disk, and so forth.

RAID-5 specifies a block-interleaved, distributed-parity disk array. InRAID-5, each entire data block is written on a data disk, and a parityblock for the corresponding data blocks in the same rank is generated.The parity blocks are recorded in locations that are distributed acrossthe disks in the array and are later verified on reads of data blocks.RAID-5 requires a minimum of three drives to implement, exhibits arelatively high read-data-transaction rate, a mediumwrite-data-transaction rate, and relatively good aggregate transferrates, and individual block data-transfer rates are about the same asthose of a single disk. High efficiency of a RAID-5 array correlateswith a low parity-disk/data-disk ratio. In RAID-5, disk failure has onlya relatively-medium impact on throughput, but rebuilding data isdifficult relative to, e.g., RAID-1. FIG. 4 illustrates the sequence ofstoring blocks in an exemplary RAID-5 distributed-parity disk array,wherein block A is written to the first disk, block B is written to thesecond disk, and then a parity block is generated by XOR-combiningblocks A and B. The parity block pAB for blocks A and B is stored on thethird disk. Block C is then written to the fourth disk, block D iswritten to the fifth disk, and then a parity block is generated byXOR-combining blocks C and D. The parity block pCD for blocks C and D isstored on the first disk. Block E is then written to the second disk,block F is written to the third disk, and so forth.

It is noted that a RAID array can implement multiple nested RAID levels,thereby conforming to the specifications of two or more RAID levels. Forexample, as shown in the exemplary RAID-1+0 (or “RAID-10”) array of FIG.5, blocks written to the disk array are mirrored and then striped. BlockA is written to the first disk, a copy A′ of block A is written to thesecond disk, block B is written to the third disk, a copy B′ of block Bis written to the fourth disk, block C is written to the first disk, acopy C′ of block C is written to the second disk, block D is written tothe third disk, a copy D′ of block D is written to the fourth disk, andso forth.

Alternatively, as shown in the exemplary RAID-0+1 array of FIG. 6,blocks written to the disk array are striped and then mirrored. Block Ais written to the first disk, block B is written to the second disk, acopy A′ of block A is written to the third disk, a copy B′ of block B iswritten to the fourth disk, block C is written to the first disk, a copyC′ of block C is written to the second disk, and so forth.

Other combinations of RAID-array levels and arrays having differentnumbers of disk drives per array are possible, and other RAIDconfigurations and levels exist (e.g., RAID-6 and RAID-50), although notspecifically mentioned or discussed herein.

As discussed above, RAID levels 1, 4, and 5 support redundancy, i.e., ifany one drive fails, the data for the failed drive can be reconstructedfrom the remaining drives. If such a RAID array is operating with asingle drive identified as failed, it is said to be operating in adegraded mode. RAID-1 and RAID-4/RAID-5 provide redundancy of data usingdifferent methods. RAID-1 provides data redundancy by mirroring, i.e.,maintaining multiple complete copies of the data in a volume. Data beingwritten to a mirrored volume is reflected in all copies, such that, if aportion of a mirrored volume fails, the system continues to use theother copies of the data. RAID-5 provides data redundancy by using thestored parity information, which is used to reconstruct data after afailure. Since parity information is calculated by performing a knownXOR procedure on data being written to a RAID-5 volume, if a portion ofa RAID-5 volume fails, the data that was on that portion of the failedvolume can be recreated by calculating the correct data using theremaining data and parity information.

Conventional RAID arrays suffer from a number of disadvantages,including the following.

RAID arrays typically use either (i) fixed-hardware implementations thatpermit a group of drives to appear as one or (ii) softwareimplementations that use the host computer's CPU to perform RAIDoperations. Disadvantageously, such traditional hardware implementationsare inflexible, and such software implementations use processor andmemory overhead. Moreover, neither permits a single set of physicaldrives to be used in more than one configuration at a time.

In conventional RAID arrays, during write operations, one sector of dataat a time is sent to various physical disks in the array, and suchtransfer of data is typically managed by software running on the hostcomputer, which calculates and provides addresses on these physicaldisks at which the data will be written. Thus, memory and processorresources of the host computer must be used.

Moreover, in such arrays, a disk controller communicates directly withphysical disks in the array. When writing to the disks, the controllermust wait for the physical disk to be ready for the write operation, orsoftware buffering by the host computer must be performed.

Additionally, during read and write operations in a conventional RAIDarray, one entire stripe is buffered at a time and stored (typically inmemory on the host computer) so that parity calculations can be made,thereby requiring substantial processor and memory resources for thiscumbersome storage and calculation process.

In conventional RAID arrays, an entire RAID array is unavailable forreading and writing while a volume is being reconstructed, andreconstruction typically involves running software on a host computerwhile all of the drives of the array are taken offline.

Another limiting aspect of conventional RAID arrays is that a user candefine only a single profile defining parameters for the set of physicaldisk drives (or other storage devices) in the array. Such arrays storeand retrieve data block-by-block, and the block size for an array istypically determined in the profile from the outset, before any data isever written to the drives. This block size does not change afterstorage to the disks has begun.

Also in the profile, traditional arrays identify disk drives as physicaldrives in the order in which they are stored in the array's physicaldrive bays (i.e., slot 0, slot 1, slot 2). The order of drives can bechanged only by physically removing, exchanging, or inserting driveswithin the drive bays. Drives can be added to a RAID array only whenthey are physically present in the array, and when drives are removedfrom the array, no configuration information for these drives is stored.Also, drive partitioning cannot be adjusted and resized on an ad-hocbasis, but, as with block size, this can only be done before the firstdata is ever written to the disks.

The drives in conventional RAID arrays are limited to a single filesystem, and there is no way for different portions of the same physicaldisk array to be used concurrently, except as part of one of theRAID-level schemes (e.g., mirroring or striping), as discussed above.

Excess capacity on disk drives in a physical disk drive array cannot beused when integrating physical drives of varying sizes into traditionalRAID arrays, and all drives in the array are limited to using only theamount of storage available on the smallest-sized drive in the array.For example, in a traditional RAID array containing three 40 GB drives,if a fourth drive of 120 GB drive is added, only 40 GB of the fourthdrive can be used.

SUMMARY OF THE INVENTION

In one embodiment, the present invention provides an apparatus forreading from a physical storage-device array including a plurality ofstorage devices. The physical storage-device array has a plurality ofsector levels, each sector level containing one sector on each storagedevice in the array at corresponding addresses on across the storagedevices. The apparatus includes: (1) a memory adapted to store two ormore profiles, each profile defining (i) a virtual array associated witha selected set of the storage devices and (ii) one or more parametersused for accessing information from the virtual array; (2) a buffer (i)having a first portion and a second portion and (ii) coupled to receivedata from the storage devices; and (3) a state machine (i) coupled tothe buffer and the memory and (ii) adapted to generate two or moresuccessive pairs of instructions. The first instruction of each pair ofinstructions (i) is based on one or more parameters of a first profileand (ii) is an instruction to read, into the first portion of thebuffer, one sector of data from each storage device of a first virtualarray; and the second instruction of each pair of instructions (i) isbased on one or more parameters of a second profile and (ii) is aninstruction to read, into the second portion of the buffer, one sectorof data from each storage device of a second virtual array.

In another embodiment, the present invention provides a method forreading from a physical storage-device array comprising a plurality ofstorage devices using a buffer having a first portion and a secondportion. The physical storage-device array has a plurality of sectorlevels, each sector level containing one sector on each storage devicein the array at corresponding addresses across the storage devices. Themethod comprises: (a) accessing first and second profiles, each profiledefining (i) a virtual array associated with a selected set of thestorage devices and (ii) one or more parameters used for accessinginformation from the virtual array; and (b) generating two or moresuccessive pairs of instructions. The first instruction of each pair ofinstructions (i) is based on one or more parameters of a first profileand (ii) is an instruction to read, into the first portion of thebuffer, one sector of data from each storage device of a first virtualarray. The second instruction of each pair of instructions (i) is basedon one or more parameters of a second profile and (ii) is an instructionto read, into the second portion of the buffer, one sector of data fromeach storage device of a second virtual array.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, features, and advantages of the present invention willbecome more fully apparent from the following detailed description, theappended claims, and the accompanying drawings in which like referencenumerals identify similar or identical elements.

FIG. 1 illustrates the sequence of storing blocks in an exemplary RAID-0striped disk array;

FIG. 2 illustrates the sequence of storing blocks in an exemplary RAID-1mirrored disk array;

FIG. 3 illustrates the sequence of storing blocks in an exemplary RAID-4block-interleaved, dedicated-parity disk array;

FIG. 4 illustrates the sequence of storing blocks in an exemplary RAID-5block-interleaved, distributed-parity disk array;

FIG. 5 illustrates the sequence of storing blocks in an exemplaryRAID-1+0 (or “RAID-10”) disk array;

FIG. 6 illustrates the sequence of storing blocks in an exemplaryRAID-0+1 disk array;

FIG. 7 illustrates an exemplary data storage system including a diskarray encoder/decoder in one embodiment of the present invention;

FIG. 8 illustrates a rotating parity-placement striping scheme in anexemplary RAID-5 five-disk array in one embodiment of the presentinvention;

FIG. 9 illustrates an exemplary data-sector addressing scheme consistentwith one embodiment of the present invention;

FIG. 10 is an interface diagram illustrating signals that the RAIDEncoder/Decoder (RDE) exchanges with the Application Processor (AAP),the Traffic Manager/Arbiter (TMA), and (iii) the Multi-Drive Controller(MDC);

FIG. 11 is a timing diagram depicting the transmission of data andcontrol signals from the Traffic Manager/Arbiter (TMA) to the RAIDEncoder/Decoder (RDE);

FIG. 12 is a frame-format diagram showing the format fortma_rde_data[31:0];

FIG. 13 is a frame-format diagram showing the format fortma_rde_data[31:0];

FIG. 14 is a timing diagram depicting the transmission of data andcontrol signals from the RAID Encoder/Decoder (RDE) to the TrafficManager/Arbiter (TMA);

FIG. 15 is a timing diagram depicting the transmission of data andcontrol signals from the Traffic Manager/Arbiter (TMA) to the RAIDEncoder/Decoder (RDE);

FIG. 16 is a frame-format diagram showing the format forrde_mdc_data[31:0] storage request frames;

FIG. 17 is a frame-format diagram showing the format forrde_mdc_data[31:0] retrieval request frames;

FIG. 18 is a timing diagram depicting the transmission of data andcontrol signals from the Multi-Drive Controller (MDC) to the RAIDEncoder/Decoder (RDE);

FIG. 19 is a frame-format diagram showing the format formdc_rde_data[31:0] retrieval response-request frames;

FIG. 20 is a frame-format diagram showing the format formdc_rde_data[31:0] storage response-request frames;

FIG. 21 is a block diagram showing the internal sub-blocks of RAIDEncoder/Decoder (RDE) of FIG. 7;

FIG. 22 is a block diagram of the Traffic-Manager Interface (TMI)sub-block of the RAID Encoder/Decoder (RDE) of FIG. 21;

FIG. 23 is a block diagram of the Write-Operation Sequencer (WOS)sub-block of the RAID Encoder/Decoder (RDE) of FIG. 21;

FIG. 24 is a state diagram depicting the operation of theWrite-Operation State Machine (WOSM) of FIG. 23;

FIG. 25 is a block diagram of the Parity-Block Processor (PBP) sub-blockof the RAID Encoder/Decoder (RDE) of FIG. 21;

FIG. 26 is a block diagram of the Write-Interface (WIF) sub-block of theRAID Encoder/Decoder (RDE) of FIG. 21;

FIG. 27 is a block diagram of the Read-Operation Sequencer (ROS)sub-block of the RAID Encoder/Decoder (RDE) of FIG. 21;

FIG. 28 is a frame-format diagram showing the format for anIssued-Request FIFO (IRF) frame;

FIG. 29 is a state diagram depicting the operation of the Read-OperationState Machine (ROSM) of FIG. 27;

FIG. 30 is a block diagram of the Read-Interface (RIF) sub-block of theRAID Encoder/Decoder (RDE) of FIG. 21;

FIG. 31 is a block diagram of the Block-Parity Reconstructor (BPR)sub-block of the RAID Encoder/Decoder (RDE) of FIG. 21; and

FIG. 32 is a block diagram of the Application-Processor Interface (AAI)and Control/Status Register (CSR) sub-blocks of the RAID Encoder/Decoder(RDE) of FIG. 21.

DETAILED DESCRIPTION

Acronyms and Abbreviations

For reference, the following is a list of acronyms and abbreviationsused herein.

AAI AAP Interface sub-block AAP Application Processor AHB AMBAHigh-Performance Bus AMBA Advanced-Microprocessor Bus Architecture BPRBlock-Parity Reconstructor sub-block CSR Control/Status Registersub-block DID Logical-Drive Identifier DMA Direct-Memory Access DSAData-Sector Address DSU Data-Sector Unit HDTV High-Definition TelevisionHTTP Hypertext-Transport Protocol IP Internet Protocol IRFIssued-Request FIFO K Chunk size LAN Local-Area Network LBALogical-Block Address ldeg Logical drive number of degraded drive LENGTHVariable Transfer Length in DSUs MDC Multi-Drive Controller PARROTParity-Rotation Index PAR_DID DID of a PSU PBP Parity-Block Processorsub-block PDID Physical-Drive Identifier PSB Parity-Sector Buffer PSUParity-Sector Unit PTC Peripheral-Traffic Controller QID Queue ID RACRAID-Array Cluster RAID Redundant Array of Independent (or Inexpensive)Disks RAID5_DID RAID-5 Logical-Drive Identifier RCFR Read-ResponseConfiguration Registers rCTL RDE-Control Register RDE RAIDEncoder/Decoder RHER Response-Header Error Register RHIBRResponse-Header Information-Buffer Register RHIR Response-HeaderInformation Register RIF Read-Interface sub-block RIRRRequest-Information Response Registers RISM Read-Interface State MachineROS Read-Operation Sequencer sub-block ROSM Read-Operation State MachineROSR Read-Operation State Registers RPSB Retrieval Parity-Sector BufferrRAC RAC-Profile Registers rRERR Error-Status Registers RTP Real-TimeTransport Protocol SATA Serial Advanced-Technology Attachment SMCShared-Memory Controller SSEQ Sector Sequencer SSU Stripe-Sector UnitSSU_DSU_OFFSET Offset of a DSA within an SSU SSUB SSU Buffer STRIPEStripe index STRIPE_DSU_OFFSET Offset of the first DSU of an SSU withina stripe TCP Transport-Control Protocol TMA Traffic Manager/Arbiter TMITraffic-Manager Interface sub-block UDP User-Datagram Protocol ULPUpper-Layer Processor USB Universal Serial Bus VAP Virtual-Array ProfileWAN Wide-Area Network WCFR Write-Request Configuration Registers WHERWrite-Header Extraction Registers WHIBR Write-Header Information BufferRegister WHIR Write-Header Information Registers WIBR Write-InformationBuffer Registers WIF Write-Interface sub-block WISM Write-InterfaceState Machine WOS Write-Operation Sequencer sub-block WOSMWrite-Operation State Machine WOSR Write-Operation State Registers WPFPending-Write Request FIFO XCNT Transfer Count in Sectors XOR ExclusiveORDefinitions

Certain terms used herein are defined as follows.

The term “disk array,” “RAID array,” “drive array,” and “array” are usedinterchangeably herein to identify a RAID array, i.e., a physical arrayof two or more disk drives, wherein the individual disks in the arrayare identified as disk0, disk1, and so forth.

The terms “disk,” “hard disk,” “drive,” “disk drive,” “hard drive,”“volume,” and “member” are used interchangeably herein to refer to astorage device in an array and should not be construed as limiting suchstorage devices to any one particular type of device (e.g., optical,magnetic, removable, etc.).

The terms “RAID-Array Cluster” (RAC), “cluster,” “array cluster,”“virtual-array cluster,” “virtual array,” and “virtual RAC” are used torefer to a virtual-RAID array, as defined by a Virtual-Array Profile(VAP) (also referred to herein as a “profile,” an “array profile,” an“RAC profile,” or “cluster profile”). In certain embodiments of theinvention, a plurality of virtual arrays exist, each having a VAP thatdefines the parameters of the virtual RAC. In prior art RAID arrays, asingle set of physical disks is defined by only a single profile thatmight be modified from time to time. However, in embodiments of theinvention employing virtual arrays, a plurality of VAPs cansimultaneously exist for a single set of physical disk drives, and thestructure and functionality of such embodiments permit more than onevirtual array for the same set of physical disks to be addressed andused substantially concurrently for read and/or write operations.

A sector is the basic unit of read and write operations and consists ofa uniquely addressable set of predetermined size, usually 512 bytes.Sectors correspond to small arcs of tracks on disk-drive platters thatmove past read/write heads on a disk as the disk rotates.

A “chunk” (also referred to herein as a “block”) is the smallest amountof data per write operation that is written to an individual disk in anarray, expressed as an integer multiple of sectors. This amount isreferred to as the array's “chunk size.” A chunk contains either parityinformation or data. The maximum chunk size in the embodiments describedherein is less than 256 sectors.

A Data-Sector Unit (DSU) is a sector's worth of data.

A Data-Sector Address (DSA) is a 32-bit numerical address that is usedto refer to a particular DSU in the array, as described below withreference to FIG. 9. In a DSA-addressing scheme, sectors are numberedsequentially from 0 to D−1, where D is the total number of DSUs in thewhole RAID array.

A Parity-Sector Unit (PSU) is a sector's worth of parity information. Ina disk array with N drives, a PSU is derived from the bit-wise XOR ofthe data in the N−1 DSUs of a Stripe-Sector Unit (SSU), as described infurther detail below.

A Logical-Block Address (LBA) is a 48-bit numerical address that is usedto refer to a sector on an individual disk drive. In an LBA-addressingscheme, sectors are numbered sequentially from 0 to S−1, where S is thetotal number of sectors on a disk drive.

A Stripe-Sector Unit (SSU) is a set of sectors that includes one sectorcollected from each drive in the array. The set of sectors in an SSUshare the same LBA, and thus, a specific SSU is referenced by the commonLBA of its member sectors. For a block-interleaved, distributed-paritydisk array with N drives, an SSU holds N−1 data sectors, plus one sectorof parity information. The term “sector level” will be used to refercollectively to the corresponding addresses of the drives at which anSSU is stored.

A stripe is a set of chunks that includes one chunk collected from eachdrive in the array. The term “stripe index” will be used to refer to anumerical address identifying a stripe within an array.

Data-Storage System Overview

FIG. 7 illustrates a data-storage system 700 in which a RAIDencoder/decoder (RDE) 701 in one embodiment of the present invention isused. While the acronym RDE (RAID Encoder/Decoder) will be used hereinto refer to block RDE 701, it should be understood that RDE 701, as wellas other components and aspects of the present invention, could be usedwith arrays of disks in which the storage of data is not actuallyredundant across disks, and that the use of the terms “RAID” and “RDE,”as used herein, is in no way meant to be construed as limiting datastorage to such redundant data storage. It should also be recognizedthat, while the following description refers generally to storage(recording) to disk and retrieval (playback) from disk of media objectscontaining audio and/or video data, e.g., for use with a personalcomputer, the invention is not limited to such objects, data, hardware,or software and may be alternatively or additionally used with othertypes of objects, data, hardware, or software.

An overview of system 700 will now be provided. System 700 includesApplication Processor (AAP) 702, network controller 703, Upper-LayerProcessor (ULP) 704, RDE 701, Multi-Drive Controller (MDC) 705,Peripheral-Traffic Controller (PTC) 706, Traffic Manager/Arbiter (TMA)707, and Shared-Memory Controller (SMC) 708. In a preferred embodiment,all of the elements of system 700 reside on a single integrated circuit.

AAP 702 may be, e.g., an ARM 32-bit RISC processor implementingassembly-language programming and running a Linux variant. It should beunderstood that, while AAP 702 receives instructions in assemblylanguage, for ease of reference herein, instructions for AAP 702, aswell as for other programmable components of system 700, will bedescribed in exemplary pseudocode and/or program code as embodied in oneor more high-level programming languages. AAP 702 is coupled to aninterface with a memory, such as FLASH memory device 709. To communicatewith each of the various blocks of system 700, AAP 702 has separateinstruction and data caches for each block, as well as separateinstruction and data-bus interfaces with each block. TMA 707 and FLASHmemory device 709 interface to AAP 702 across instruction interfaces anddata interfaces, because both TMA 707 and FLASH memory device 709contain instructions and data used by AAP 702. FLASH memory device 709may contain, e.g., boot code for AAP 702 to permit configuration ofvarious registers of system 700.

Network controller 703 may be, e.g., a Gigabit Ethernet (1000Mbits/second) controller and is coupled to interface with a standardnetwork 710, such as an external LAN or WAN or the Internet, forcommunicating with various computers and other devices.

ULP 704 implements, in hardware, upper-layer protocol-processingfunctionality for system 700. Such processing may include, e.g.,Internet-layer (Layer 3), transport-layer (Layer 4), andapplication-layer (Layer 5 and above) processing and may implement oneor more protocol types including, e.g., Internet Protocol (IP) (version4 and version 6), Transport-Control Protocol (TCP), User-DatagramProtocol (UDP), Real-time Transport Protocol (RTP), andHypertext-Transport Protocol (HTTP).

RDE 701 performs RAID encoding and decoding of data in shared memory 711for storage onto and retrieval from a multiple-disk array that includeshard drives 712, e.g., between 3 and 8 Serial-AT Attachment (SATA)drives. RDE 701 translates between (i) LBAs, which refer to sectors ofindividual disk drives 712, and (ii) DSAs and SSUs, which refer tosectors with respect to the entire array, as will be explained infurther detail below.

MDC 705 provides a point-to-point multiple-independent channelinterface, e.g., a high-speed unidirectional SATA interface employingDirect-Memory Access (DMA) for storage and retrieval operations andsupporting first-generation data rates of 1.5 Gbps (150 MB/s), throughwhich RDE 701 stores data on one or more of hard disks 712, e.g., in aRAID configuration. For a write operation, a starting LBA is generatedby RDE 701 and provided to MDC 705 along with DSUs to be written. MDC705 formats the received data into frames, encodes the data, appends aCRC, and serializes the data for transmission across the SATA interface.For a read operation, MDC 705 deserializes and decodes received data tobe read and checks the received data for integrity prior to transferringthe received data to RDE 701. To minimize latency, RDE 701simultaneously distributes data being written to or read from drives 712of the array, one stripe (set of chunks) at a time. MDC 705 alsointerfaces with AAP 702 for read/write access to command and controlregisters residing in a SATA controller (not shown) included in MDC 705.

PTC 706 enables system 700 to communicate with external devices over aUniversal Serial Bus (USB) interface and employs a USB controllercoupled to ULP 704, TMA 707, and AAP 702. PTC 706 provides a single USBcore that can function as either a host or a peripheral USB device.Thus, in addition to using network controller 703 (e.g., an Ethernetport) to connect with networked devices, system 700 can also use PTC 706to provide an additional mechanism to connect with external devices,e.g., a USB-based Wireless-LAN adapter, a media read/write device, adigital camera, or a remote control device.

TMA 707 manages (i) media traffic arriving from network 710 for storage,(ii) control traffic arriving for processing by AAP 702, and (iii)playback traffic during retrieval from storage. TMA 707 includes abuffer manager and a scheduler. The buffer manager allocates andde-allocates buffers during the media object re-assembly process, theplayback process, and accesses of AAP 702. The scheduler manages sharedresources, such as memory-access bandwidth and disk-access bandwidth,and provides guaranteed bandwidth and latency guarantees for mediaobjects during playback.

SMC 708 may be a DDR II memory controller and is coupled to interfacewith one or more external shared memories 711, such as standard DDR IISDRAM devices. PTC 706 is coupled to interface with an external datainterface, such as a USB interface 713 of a personal computer 714. MDC705 is coupled to interface simultaneously with multiple hard disks 712in the storage-device array, over a plurality of independent channels,one channel per hard disk.

AAP 702, network controller 703, ULP 704, RDE 701, MDC 705, PTC 706, TMA707, and SMC 708 are coupled to interface with one another via a databus, e.g., an Advanced-Microprocessor Bus Architecture (AMBA) High-Speedbus (AHB). In addition to being coupled via the AHB data bus, PTC 706,TMA 707, and ULP 704 are coupled to interface with one another viaanother data bus, e.g., a USB bus. TMA 707 and AAP 702 are also coupledto interface with one another via an instruction bus, e.g., an AHBinstruction bus.

As illustrated by the various directional arrows in FIG. 7, system 700has two separate data paths: (i) a receive path, which is the directionby which traffic flows from external devices to system 700, and (ii) atransmit path, which is the direction by which traffic flows from system700 to external devices. Packet-based transfers flow through ULP 704 (i)to and from network controller 703 and (ii) to and from PTC 706.Non-packet-based transfers flow directly between PTC 706 and TMA 707 viathe USB bus.

In the receive path, one or both of network controller 703 and PTC 706receives packets (e.g., Ethernet packets) from a physical interface.Network controller 703 performs various protocol-related checking, e.g.,packet-integrity verification and multicast-address filtering. Thepackets are then passed to ULP 704 for further processing. Such furtherprocessing may include, e.g., extracting and parsing Layer-2, Layer-3,and Layer-4 header fields to form an address and performing a lookupbased on the address. Using the lookup result, ULP 704 decides where tosend the received packet. A packet arriving over an already-establishedconnection is tagged with a pre-defined Queue ID (QID), which is used byTMA 707 for traffic-queuing purposes.

A packet arriving over a connection that has not yet been established,e.g., from an unknown connection, is tagged with a special QID and isrouted to AAP 702 for further investigation. The final destination of apacket that has arrived and is processed by AAP 702 will be either oneor more of hard disks 712 for storage, e.g., if the packet carries mediacontent, or TMA 707 for further investigation, e.g., if the packetcarries a control message or cannot be recognized by AAP 702. TMA 707stores arriving packets in shared memory 711. If a packet contains anincoming media object, the incoming media object data is stored inshared memory 711 and is transferred to RDE 701 for storage on one ormore of hard disks 712. TMA 707 manages the storage process by providingappropriate control information to RDE 701. Packets, such as controlmessages, that are destined for inspection by AAP 702 are stored inshared memory 711 as well, and AAP 702 has access to read stored packetsout of shared memory 711 and to write packets to shared memory 711. AAP702 is also configured to use this read/write access to shared memory711 to re-order any packets that were received out of order.

A portion of shared memory 711 and hard disks 712 contains programinstructions and data for AAP 702. TMA 707 manages access to sharedmemory 711 and hard disks 712 by transferring control informationbetween shared memory 711 and hard disks 712. TMA 707 also enables AAP702 to insert data into and extract data from an existing packet stream.RDE 701 encodes data blocks from shared memory 711 and writes theencoded data blocks onto one or more of hard disks 712 via MDC 705.

In the transmit path, TMA 707 manages requests to retrieve, from one ormore of hard disks 712, objects that are destined to AAP 702 or networkcontroller 703. Upon receiving a media-playback request from AAP 702,TMA 707 receives the media-object data transferred from one or more ofhard disks 712 through MDC 705 and RDE 701 and stores the received datain shared memory 711. TMA 707 then schedules the data to ULP 704,according to (i) the type of media (e.g., audio or video) stored thereinand (ii) the expected bandwidth requirements for the media object. Foreach outgoing packet, ULP 704 encapsulates the data, e.g., with Ethernetand Layer-3/Layer-4 headers. The packets are then routed, based on thedestination port specified, either to network controller 703 (e.g., forEthernet packets) or to PTC 706 (e.g., for packets exchanged via USBinterface 713).

RDE Parity-Placement, Data-Sector Addressing, and Calculations

Given the context of exemplary system 700 in which RDE 701 is used, theparticular structure and function of RDE 701 will now be described infurther detail, beginning with an explanation of the parity-placementscheme, data-sector addressing scheme, and calculations used by RDE 701in one embodiment of the invention.

FIG. 8 illustrates a rotating parity-placement striping scheme employedby RDE 701 in an exemplary RAID-5 array in one embodiment of the presentinvention. Lowercase letters represent chunks of data stored on disk0through disk4 as follows. The following five chunks are stored at thesame time: Chunk a is stored on disk0, chunk b is stored on disk1, chunkc is stored on disk2, chunk d is stored on disk3, and parity chunk P0(which was generated by XOR-combining chunks a, b, c, and d) is storedon disk4. Next, the following five chunks are stored at the same time:chunk e is stored on disk0, chunk f is stored on disk1, chunk g isstored on disk2, parity chunk P1 (which was generated by XOR-combiningchunks e, f, g, and h) is stored on disk3, and chunk h is stored ondisk4. Next, the following five chunks are stored at the same time:chunk i is stored on disk0, chunk j is stored on disk1, parity chunk P2(which was generated by XOR-combining chunks i, j, k, and l) is storedon disk2, chunk k is stored on disk3, and chunk l is stored on disk4.Next, the following five chunks are stored at the same time: chunk m isstored on disk0, parity chunk P3 (which was generated by XOR-combiningchunks m, n, o, and p) is stored on disk1, chunk n is stored on disk2,chunk o is stored on disk3, and chunk p is stored on disk4. Next, thefollowing five chunks are stored at the same time: parity chunk P4(which was generated by XOR-combining chunks q, r, s, and t) is storedon disk0, chunk q is stored on disk1, chunk r is stored on disk2, chunks is stored on disk3, and chunk t is stored on disk4. Next, thefollowing five chunks are stored at the same time: chunk u is stored ondisk0, chunk v is stored on disk1, chunk w is stored on disk2, chunk xis stored on disk3, and then parity chunk P5 (which was generated byXOR-combining chunks u, v, w, and x) is stored on disk4, and so forth.In this scheme, parity rotation through data is by stripes of chunks. Inother words, in this rotating-parity scheme, parity chunks aredistributed in round-robin manner across the drives of the disk arrayand through the data chunks of the stripes, such that each stripecontains exactly one parity chunk, and each subsequent stripe contains aparity chunk in a position that is “left-rotated” from that of theparity chunk of the current stripe. It should be understood thatalternative parity placements are possible in various embodiments of thepresent invention. For example, alternative embodiments could employ aright-symmetric parity scheme or a different parity scheme wherein onechunk of parity per stripe is written.

FIG. 9 illustrates an exemplary DSA data-sector addressing schemeemployed by RDE 701 in one embodiment of the present invention. Asshown, the individual data sectors on disk0 through disk4 are numberedsequentially from 0 to D−1, where D is the total number of DSUs in theRAID array. PSUs P0, P1, and so forth, each of which contains a sector'sworth of parity information, are not included in the DSA-addressingscheme, i.e., the sequential numbering is not advanced for PSUs. Forexample, the set of sectors having DSAs numbered [1, 5, 9, 13]constitutes a data chunk on disk1, whereas the set of sectors labeled[P0, P1, P2, P3] constitutes a parity chunk on disk4. Within eachstripe, this DSA scheme advances from one disk to the next by SSU,rather than by chunk.

The foregoing DSA-addressing scheme permits logical translations betweenDSA and LBA addresses. The LBA of an SSU can be obtained by dividing theDSA by one less the number N of drives in the array. The remainder(SSU_DSU_OFFSET) is the offset of the DSA within an SSU. Thus:

-   -   LBA=DSA/(N−1); and    -   SSU_DSU_OFFSET=DSA mod (N−1).        The stripe index (STRIPE) can be obtained by dividing the DSA by        the product of the chunk size (K) and one less the number of        drives in the array, with the remainder from the division being        the offset in DSUs from the beginning of the stripe. The        STRIPE_DSU_OFFSET is the offset of the first DSU of an SSU        within a stripe. Thus:    -   STRIPE=DSA/(K*(N−1));    -   STRIPE_DSU_OFFSET=DSA mod (K*(N−1));    -   STRIPE_SSU_OFFSET=STRIPE_DSU_OFFSET−SSU_DSU_OFFSET; and    -   SSU_OF_STRIPE=STRIPE_SSU_OFFSET/(N−1).        The Parity-Rotation Index (PARROT), which represents the number        of disks through which to rotate beginning from the left-most        disk, is the result of modulo division of the stripe index by        the number of drives in the array. The Parity-Rotation Index        ranges from 0 to one less than the number of drives in the        array. Thus:    -   PARROT=STRIPE mod N; and    -   keep PARROT in [0 . . . N−1].

Logical-Drive Identifiers, also referred to herein as DIDs, arenumerical identifiers used in operations that specify particular logicalmembers (i.e., disk drives) of an array. DIDs range from 0 to one lessthan the number of drives in the array. Thus:

-   -   keep DID in [0 . . . N−1].        RDE 701 is capable of handling encoding and decoding operations        for both RAID-4 and RAID-5 disk arrays. Since RAID-4 ignores        parity rotation, a RAID-4 DID of a DSA within an SSU (RAID4_DID)        is the remainder of the division of the DSA by the number of        drives in the array. Thus:    -   RAID4_DID=DSA mod (N−1).        A PSU's DID (PAR_DID) is one less than the number of disk drives        in the array less the Parity-Rotation Index. Thus:    -   PAR_DID=(N−PARROT−1).        A RAID-5 DID is the RAID-4 DID, adjusted for parity rotation.        Thus:    -   if (RAID4_DID<PAR_DID)    -   then    -   RAID5_DID=RAID4_DID    -   else    -   RAID5_DID=RAID4_DID+1    -   fi.        Given a Parity-Rotation Index and a RAID-5 DID, the        corresponding logical RAID-4 DID can be obtained, as follows:    -   if (RAID5_DID==(N−PARROT−1))//PAR_DID?    -   then    -   RAID4_DID=N−1    -   elsif (RAID5_DID<(N−PARROT−1))    -   RAID4_DID=RAID5_DID    -   else    -   RAID4_DID=RAID5_DID−1    -   fi

Physical-Drive Identifiers (PDIDs) are numerical identifiers identifyingthe actual physical drives in the disk array. The mapping of a RAID5_DIDto the corresponding PDID is stored in the array's VAP, as stored inRAC-Profile Registers (rRAC), which are described in further detailbelow (and shown in Tables 21-25).

TMA 707 provides a variable transfer length (LENGTH), expressed as thenumber of DSUs that are to be distributed over the array. For dataretrieval, any non-zero offset is added to LENGTH in order to retrieveentire SSUs. This per-drive offset is the operative number of SSUs perdrive, which number is obtained by dividing the sum of LENGTH and theoffset by one less than the number of drives in the array, and thenrounding the quotient up. This Transfer Count (XCN7), expressed insectors, is provided to MDC 705 for each of the disks in the array.Thus:

-   -   if ((LENGTH+SSU_DSU_OFFSET) mod (N−1)=0)    -   then    -   XCNT=(LENGTH+SSU_DSU_OFFSET)/(N−1)    -   else    -   XCNT=((LENGTH+SSU_DSU_OFFSET)/(N−1))+1    -   fi.        RDE Interface with AAP, TMA, and MDC

FIG. 10 is an interface diagram illustrating signals that RDE 701exchanges with (i) AAP 702, (ii) TMA 707, and (iii) MDC 705.

As core signals, AAP 702 provides to RDE 701 (i) 1-bit pulse signalreset_cc_n, which enables a global reset of RDE 701, and (ii) 1-bitclock signal core_clk, which is the basic core clock pulse fed to allcomponents of system 700 and is desirably nominally 125 mHz.

To control data flow from TMA 707 to RDE 701, the following signals areexchanged. TMA 707 provides to RDE 701 (i) 32-bit data signaltma_rde_data[31:0], which contains data and/or control dwords (dataelements), (ii) 1-bit control signal tma_rde_soh, which is used to marka start of header (SOH) on tma_rde_data[31:0], and (iii) 1-bit controlsignal tma_rde_valid, which indicates whether signals tma_rde_data[31:0]and tma_rde_soh are valid. RDE 701 provides to TMA 707 1-bit controlsignal rde_tma_ready, which indicates whether RDE 701 is ready to accepttma_rde_data[31:0].

To control data flow from RDE 701 to TMA 707, the following signals areexchanged. RDE 701 provides to TMA 707 (i) 32-bit data signalrde_tma_data[31:0], which contains data and/or control dwords, (ii)1-bit control signal rde_tma_soh, which is used to mark an SOH onrde_(tma)_data[31:0], and (iii) 1-bit control signal rde_tma_valid,which indicates whether signals rde_tma_data[31:0] and rde_tma_soh arevalid. TMA 707 provides to RDE 701 1-bit control signal tma_rde_ready,which indicates whether RDE 701 is ready to accept rde_tma_data[31:0].

To control data flow from RDE 701 to MDC 705, the following signals areexchanged. RDE 701 provides to MDC 705 (i) 32-bit data signalrde_mdc_data[31:0], which contains data and/or control dwords, (ii)1-bit control signal rde_mdc_soh, which is used to mark an SOH onrde_mdc_data[31:0], (iii) 1-bit control signal rde_mdc_valid, which isused to indicate whether signals rde_mdc_data[31:0] and rde_mdc_soh arevalid, and (iv) 3-bit control signal rde_mdc_wdid[2:0], which providesthe PDID for the disk in the array to which data is to be written. MDC705 provides to RDE 701 8-bit data signal mdc_rde_ready[7:0], whichindicates, for each disk in the array, whether MDG 705 is ready toaccept rde_mdc_data[31:0].

To control data flow from MDC 705 to RDE 701, the following signals areexchanged. MDC 705 provides to RDE 701 (i) 32-bit data signalmdc_rde_data[31:0], which contains data and/or control dwords, (ii)1-bit control signal mdc_rde_soh, which is used to mark an SOH onmdc_rde_data[31:0], (iii) 1-bit control signal mdc_rde_valid, which isused to indicate whether signals mdc_rde_data[31:0], mdc_rde_soh, andmdc_rde_rdid[2:0] are valid, and (iv) 3-bit control signalmdc_rde_rdid[2:0], which provides the PDID for the disk in the arrayfrom which a valid retrieval operation is taking place. RDE 701 providesto MDC 705 (i) 3-bit control signal rde_mdc_rdid[2:0], which indicatesthe requested PDID for a retrieval operation and (ii) 1-bit controlsignal rde_mdc_ready, which indicates that RDE 701 is ready to acceptmdc_rde_data[31:0] from the drive indicated by rde_mdc_rdid[2:0].

To control data flow between RDE 701 and AAP 702, the following signalsare exchanged. AAP 702 provides to RDE 701 (i) 32-bit data signalaap_hwdatad[31:0], which contains data being provided by AAP 702 to RDE701 during a write operation to a register of RDE 701 stored inControl/Status Registers (CSR) 2108, (ii) 28-bit data signalaap_haddrd[27:0], which contains the address of a register in CSR 2108specified by AAP 702 for the write operation, (iii) 2-bit data signalaap_htransd[1:0], which indicates the type of the current transfer andcan be NONSEQUENTIAL, SEQUENTIAL, IDLE, or BUSY, (iv) 1-bit controlsignal aap_rde_hseld, which is a slave-select bit (each slave device onthe AHB bus has its own slave select signal, and signal aap_rde_hseldindicates that the current transfer is intended for the selected slave),and (v) 1-bit control signal aaphwrited, which indicates whether thecurrent operation is a read operation (aap_hwrited=0) or a writeoperation (aap_hwrited=1). RDE 701 provides to AAP 702 (i) 1-bit controlsignal rde_aap_hreadyd and (ii) rde_aap_hrespd, both of which arehandshake control signals, (iii) 32-bit data signalrde_aap_hrdatad[31:0], which contains data being provided to AAP 702from RDE 701 during a read operation from a register of RDE 701 storedin CSR 2108, (iv) 1-bit control signal rde_aap_inth, which is ahigh-priority interrupt request by RDE 701 to AAP 702, and (v) 1-bitcontrol signal rde_aap_intl, which is a low-priority interrupt requestby RDE 701 to AAP 702.

With reference to the timing diagram of FIG. 11, the transmission ofdata and control signals from TMA 707 to RDE 701 will now be discussed.Signal tma_rde_soh marks SOH control information that shares theTMA-to-RDE interface with data, and signal tma_rde_data[31:0] containsthe control information and data. At transition 1, RDE 701 indicatesthat it is ready for a transfer from TMA 707 by assertion ofrde_tma_ready. At transition 2 (after a fixed multi-cycle delay), TMA707 (i) recognizes the ready status, (ii) asserts tma_rde_valid (if ithas valid data to send), (iii) asserts tma_rde_soh, which marks SOHcontrol information that shares the tma_rde_data[31:0] interface withdata, and (iv) presents data/control information via signaltma_rde_data[31:0]. RDE 701 recognizes and accepts any validdata/control information. At transition 3, RDE 701 requests a pause bydeassertion of rde_tma_ready. At transition 4 (after a fixed multi-cycledelay), information transfer pauses. At transition 5, RDE 701 indicatesthat it is ready to continue by assertion of rde_tma_ready. Attransition 6 (after a fixed multi-cycle delay), the information transfercontinues with the transmission of data on tma_rde_data[31:0] and theassertion of tma_rde_valid, and so forth.

It is noted that the number of cycles of tma_rde_valid assertion is lessthan or equal to the number of cycles for which signal rde_tma_ready wasasserted. Signal tma_rde_valid is only asserted in a cycle-by-cycleresponse to an rde_tma_ready assertion, and the multi-cycle delay of thetma_rde_valid response to rde_tma_ready is fixed.

FIG. 12 is a frame-format diagram showing the format for atma_rde_data[31:0] storage-request frame, and FIG. 13 is a frame-formatdiagram showing the format for a tma_rde_data[31:0] retrieval-requestframe. As shown, words 0 and 1 contain the same fields for both storagerequests and retrieval requests. Word 0 includes the following fields.At bit [31], field T indicates the type of request, as instructed by TMA707, and is 0 for a storage request (i.e., the data field contains data)and 1 for a retrieval request. At bits [30:24], field QID[6:0] is aqueue identifier containing the QID for which the data is beingretrieved or stored. At bits [23:20], field RAC[3:0] indicates whichRAID-Array Cluster is to be operative for the transfer. At bits [19:4],field LENGTH[ 15:0] indicates the number of sectors of the contiguouslength of the transfer (since sixteen bits are allocated to LENGTH, inunits of sectors, transfers can be up to 64 k sectors, i.e., or 32megabytes, in this embodiment). At bits [3:0] of word 0 and bits [31:0]of word 1, field DSA[35:0] indicates the DSA of the starting DSU toaccess. Words 2 through (LENGTH*128)+1 consist of field DATA[31:0],which contains the user data being transferred. The DATA[31:0] field isonly present during a storage operation, i.e., if field T has a value of0, and only words 0 and 1 are transmitted during a retrieval operation,i.e., if field T has a value of 1.

With reference to the timing diagram of FIG. 14, the transmission ofdata and control signals from RDE 701 to TMA 707 will now be discussed.Signal rde_tma_soh marks SOH control information that shares theRDE-to-TMA interface with data, and signal rde_tma_data[31:0] containsthe control information and data. At transition 1, TMA 707 indicatesthat it is ready for a transfer from RDE 701 by assertion oftma_rde_ready. At transition 2 (after a fixed multi-cycle delay), RDE701 (i) recognizes the ready status, (ii) asserts rde_tma_valid (if ithas valid data to send), (iii) asserts rde_tma_soh, which marks SOHcontrol information that shares the rde_tma_data[31:0] interface withdata, and (iv) presents data/control information via signalrde_tma_data[31:0]. TMA 707 recognizes and accepts any validdata/control information. At transition 3, TMA 707 requests a pause bydeassertion of tma_rde_ready. At transition 4 (after a fixed multi-cycledelay), information transfer pauses. At transition 5, TMA 707 indicatesthat it is ready to continue by assertion of tma_rde_ready. Attransition 6 (after a fixed multi-cycle delay), the information transfercontinues with the transmission of data on rde_tma_data[31:0] and theassertion of rde_tma_valid, and so forth.

It is noted that the number of cycles of rde_tma_valid assertion is lessthan or equal to the number of cycles for which signal tma_rde_ready wasasserted. Signal rde_tma_valid is only asserted in a cycle-by-cycleresponse to a tma_rde_ready assertion, and the multi-cycle delay of therde_tma_valid response to tma_rde_ready is fixed.

The frame format for a tma_rde_data[31:0] storage-request frame issubstantially the same as the frame format for an rde_tma_data[31:0]storage-request frame and is provided in FIG. 12. Likewise, the frameformat for a tma_rde_data[31:0] retrieval-request frame is substantiallythe same as the frame format for an rde_tma_data[31:0] retrieval-requestframe and is provided in FIG. 13.

With reference to the timing diagram of FIG. 15, the transmission ofdata and control signals from RDE 701 to MDC 705 will now be discussed.Signal rde_mdc_soh marks SOH control information that shares theRDE-to-MDC interface with data, and signal rde_mdc_data[31:0] containsthe control information and data. At transition 1, RDE 701 specifies aPDID for the transfer via signal rde_mdc_wdid[2:0]. At transition 2(after a fixed multi-cycle delay), MDC 705 indicates that its FIFObuffer corresponding to the selected PDID has space available byassertion of signal mdc_rde_ready[7:0], whose bit mapping corresponds tothe selected PDID. At transition 3 (after a fixed multi-cycle delay),RDE 701 (i) recognizes the ready status, (ii) asserts rde_mdc_valid (ifit has valid data queued to send), (iii) asserts rde_mdc_soh, whichmarks SOH control information that shares the rde_mdc_data[31:0]interface with data, and (iv) presents data/control information viasignal rde_mdc_data[31:0]. MDC 705 recognizes and accepts any validdata/control information and steers it with the address corresponding tothe selected PDID identified by rde_mdc_wdid[2:0]. At transition 4, MDC705 indicates, by deasserting the corresponding mapped bit ofmdc_rde_ready[7:0], that its FIFO buffer corresponding to the selectedPDID is almost full. At transition 5 (after a fixed multi-cycle delay),information transfer pauses. At transition 6 (after a fixed multi-cycledelay), RDE 701 specifies an alternate PDID on signal rde_mdc_wdid[2:0].At transition 7, MDC 705 indicates, by assertion of mdc_rde_ready[7:0],that its FIFO buffer corresponding to the alternate PDID selected byrde_mdc_wdid[2:0] has space available. At transition 8 (after a fixedmulti-cycle delay), the previously paused information transfer continueswith the transmission of data on rde_mdc_data[31:0] and the assertion ofrde_mdc_valid, and so forth.

It is noted that the number of cycles of rde_mdc_valid assertion is lessthan or equal to the number of cycles for which signalmdc_rde_ready[7:0] was asserted. Signal rde_mdc_valid is only assertedin a cycle-by-cycle response to a chosen mdc_rde_ready[7:0] assertion,and the multi-cycle delay of the rde_mdc_valid response tomdc_rde_ready[7:0] status is fixed.

To prevent blocking, when a PDID is selected for which an unclearederror-status bit is set in the Error-Status Registers (rRERR), which arediscussed in further detail below, RDE 701 will regard the state of thatcorresponding bit of the ready (almost-full) status bus as being ready,regardless of its actual state.

FIG. 16 is a frame-format diagram showing the format for anrde_mdc_data[31:0] storage-request frame, and FIG. 17 is a frame-formatdiagram showing the format for an rde_mdc_data[31:0] retrieval-requestframe. As shown, words 0, 1, and 2 contain the same fields for bothstorage requests and retrieval requests. Word 0 includes the followingfields. At bit [31], field T indicates the type of request, asinstructed by RDE 701, and is 0 for a storage request (i.e., the datafield contains data) and 1 for a retrieval request. At bits [30:24],field QID[6:0] is a queue identifier containing the QID for which thedata is being retrieved or stored. Bits [23:16] are not used in thisembodiment. At bits [15:0], field XCNT[15:0] indicates the transfercount, in sectors. It should be noted that field XCNT[15:0] is not thesame as the LENGTH[15:0] field of tma_rde_data[31:0]. LENGTH[15:0] isspecified in units of data sectors and represents the data that is to betransferred between RDE 701 and TMA 707, which RDE 701 spreads over theentire array. The XCNT[15:0] field, on the other hand, is drive-specificand can include data and parity information that is not transferredbetween RDE 701 and TMA 707. Bits [31:15] of word 1 are not used in thisembodiment. At bits [15:0] of word 1 and bits [31:0] of word 2, fieldLBA[47:0] indicates an LBA identifying the starting sector address forthe storage or retrieval operation. Words 3 through (XCNT*128)+2 consistof field DATA[31:0], which contains the user data being transferred. TheDATA[31:0] field is only present during a storage operation, i.e., iffield T has a value of 0, and only words 0, 1, and 2 are transmittedduring a retrieval operation, i.e., if field T has a value of 1.

With reference to the timing diagram of FIG. 18, the transmission ofdata and control signals from MDC 705 to RDE 701 will now be discussed.Signal mdc_rde_soh marks SOH control information that shares theMDC-to-RDE interface with data, and signal mdc_rde_data[31:0] containsthe control information and data. At transition 1, RDE 701 indicatesthat it is ready for a transfer from MDC 705 by (i) specifying arequested PDID for the transfer via signal rde_mdc_rdid[2:0] and (ii)asserting signal rde_mdc_ready. At transition 2 (after a fixedmulti-cycle delay), MDC 705 (i) recognizes the ready status, (ii)asserts mdc_rde_valid (if it has valid data queued to send), (iii)asserts mdc_rde_soh, which marks SOH control information that shares themdc_rde_data[31:0] interface with data, (iv) presents data/controlinformation via signal mdc_rde_data[31:0], and (v) specifies, via signalrde_mdc_rdid[2:0], the PDID of the drive from which the data/controlinformation is being provided. RDE 701 recognizes and accepts any validdata/control information. At transition 3, RDE 701 indicates, bydeasserting rde_mdc_ready, that its read-FIFO buffer (in read-FIFObuffers 3000 of RIF 2105) corresponding to the selected PDID is nolonger ready to receive. At transition 4 (after a fixed multi-cycledelay), information transfer pauses. At transition 5 (after a fixedmulti-cycle delay), RDE 701 (i) specifies a first alternate PDID onrde_mdc_rdid[2:0] and (ii) indicates it is ready to receive again bycontinued assertion of rde_mdc_ready. At transition 6 (after a fixedmulti-cycle delay), the previously-paused information transfercontinues, i.e., MDC 705 (i) recognizes the ready status, (ii) assertsmdc_rde_valid, (ii) presents data/control information via signalmdc_rde_data[31:0], and (iii) specifies, via signal rde_mdc_rdid[2:0],the PDID of the first alternate drive from which the data/controlinformation is now being provided, and RDE 701 specifies a secondalternate PDID on rde_mdc_rdid[2:0]. At transition 7 (after a fixedmulti-cycle delay), information transfer continues from the secondalternate PDID that was specified on rde_mdc_rdid[2:0], i.e., MDC 705(i) presents data/control information via signal mdc_rde_data[31:0] and(ii) specifies, via signal rde_mdc_rdid[2:0], the PDID of the secondalternate drive from which the data/control information is now beingprovided.

It is noted that the number of cycles of mdc_rde_valid assertion is lessthan or equal to the number of cycles for which signal rde_mdc_ready wasasserted. Signal mdc_rde_valid is only asserted in a cycle-by-cycleresponse to a chosen rde_mdc_ready assertion, and the multi-cycle delayof the mdc_rde_valid response to rde_mdc_ready status is fixed.

FIG. 19 is a frame-format diagram showing the format for anmdc_rde_data[31:0] retrieval-response frame, and FIG. 20 is aframe-format diagram showing the format for an mdc_rde_data[31:0]storage-response frame. As shown, words 0, 1, and 2 contain the samefields for both retrieval-response frames and storage-response frames.Word 0 includes the following fields. At bit [31], field T indicates thetype of request, as instructed by MDC 705, and is 0 for a storagerequest and 1 for a retrieval request (i.e., the data field containsdata). At bits [30:24], field QID[6:0] is a queue identifier containingthe QID for which the data is being retrieved or stored. At bit [23],field E indicates an error condition, which MDC 705 uses to terminate aretrieval operation. Bits [22:0] are not used in this embodiment. Words1 through (XCNT*128) consist of field DATA[31:0], which contains theuser data being transferred. The DATA[31:0] field is only present duringa retrieval operation, i.e., if field T has a value of 1, and only word0 is transmitted during a retrieval operation, i.e., if field T has avalue of 0.

Internal RDE Structure

FIG. 21 is a block diagram showing the internal sub-blocks of RDE 701and data flow within RDE 701, in one embodiment of the invention. Asshown, the sub-blocks of RDE 701 include Traffic-Manager Interface (TMI)2100, Write-Operation Sequencer (WOS) 2101, Parity-Block Processor (PBP)2102, Write Interface (WIF) 2103, Read-Operation Sequencer (ROS) 2104,Read Interface (RIF) 2105, Block-Parity Reconstructor (BPR) 2106, AAPInterface (AAI) 2107, and Control/Status Registers (CSR) 2108.

While not specifically shown in FIG. 21 as coupled to other sub-blocksof RDE 701, AAP interface (AAI) 2107 is common to all of the sub-blocksof RDE 701 and exchanges data and control signals between AAP 702 andthe various sub-blocks of RDE 701. AAI 2107 also provides access to CSR2108, which are memory-mapped processor-accessible registers andmemories that are used by the various sub-blocks of RDE 701.

As shown in FIG. 21, an overview of the top-level data and control flowis as follows. Requests for data transfers, including both storage andretrieval, are pulled as frames from TMA 707 through TMI 2100. TheTMI-TMA interface handshake is flow-control provisioned.

As discussed above, an SOH marks the first dword of header informationthat begins each request frame. For retrieval, only header informationis present, but for storage, data to be stored follows headerinformation. Both types of headers generally share the same format,undergo the same translations, and trace the same route towards MDC 705.

Translated header information is conveyed to WIF 2103 for distributionand to PBP 2102 for initialization.

Data to be stored in shared memory 711 passes from TMI 2100 through PBP2102 to WIF 2103, which passes the data to MDC 705 for storage on drives712 of the array.

PBP 2102 performs block-parity generation on sectors from file-systemchunks and maps data and parity to SSUs. WIF 2103 providesFIFO-buffering and an interface for de-multiplexing to disk drives 712in the array. WOS 2101 copies storage and retrieval requests to(Issued-Request FIFO (IRF) 2700 of) ROS 2104, while WIF 2103 writes toFIFOs of MDC 705 that correspond to disk drives 712.

ROS 2104 awaits completion of issued requests of which ROS 2104 wasnotified by WOS 2101.

Data read from response FIFOs of MDC 705 passes through RIF 2105 to BPR2106, for transfer to TMA 707 through TMI 2100.

BPR 2106 reconstructs missing data for a degraded array using intactstriped data and parity information.

System 700 desirably supports eight simultaneous High-DefinitionTelevision (HDTV) channels of 20 Mbits/second each, for an aggregatebandwidth of 160 Mbits/second. Network controller 703 desirably supportstraffic up to 1 gigabit/second in each direction. Accordingly, RDE 701is desirably capable of providing enough bandwidth to saturate theinterface between RDE 701 and network controller 703. Each of harddrives 712 is desirably a SATA drive capable of interface-transfer ratesof 150 Megabytes/second. For a RAID-5 array of N disk drives, theaggregate interface-transfer rate is thus (N−1)*150 Megabytes/second, orfrom 300 Megabytes/second (for a 3-drive array) to 1050 Megabytes/second(for an 8-drive array). This interface-transfer rate is an upper boundto the capability of drives 712. A 32-bit wide path interface enters RIF2105, and a 32-bit wide path interface exits RIF 2105. Likewise, a32-bit wide path interface enters WIF 2103, and a 32-bit wide pathinterface exits WIF 2103. At 125 mHz (disregarding flow control), theseinterfaces therefore each should have an upper limit of 4000Mbits/second, for an aggregate upper limit of eight gigabits/second. A32-bit wide read-path interface enters TMI 2100, and a 32-bit wideread-path interface exits TMI 2100. Likewise, a 32-bit wide write-pathinterface enters TMI 2100, and a 32-bit wide write-path interface exitsTMI 2100. At 125 mHz (disregarding flow-control), these interfacestherefore each should have an upper limit of 4000 Mbits/second, for anaggregate upper limit of eight gigabits/second. TMA 707 desirably has anupper limit of 1.26 gigabits/second bandwidth that it can devote to itsinterface with RDE 701 in each direction.

FIG. 22 illustrates Traffic-Manager Interface (TMI) sub-block 2100 ofRDE 701. As shown, TMI includes response FIFO 2200 (e.g., a 2 k×33-bitFIFO), Write-Information Buffer Registers (WIBR) 2201, Read-InterfaceState Machine (RISM) 2202, and Write-Interface State Machine (WISM)2203. TMI 2100 interfaces to TMA 707, which controls access to sharedmemory 711 of AAP 702. In the read path, in response to demands from TMA707, response FIFO 2200 receives data from BPR 2106 on 33-bit datasignal bpr_data[32:0], and data is read out of response FIFO 2200 to TMA707 on 32-bit data signal rde_tma_data[31:0], as controlled by RISM2202. In the write path, in response to demands from TMA 707, WIBR 2201receives data from TMA 707 on 32-bit data signal tma_rde_data[31:0], anddata is read out of WIBR 2201 to PBP 2102 (for parity generation) on32-bit data signal pbp_indata[31:0], as controlled by WISM 2203. WIBR2201 also provides to PBP 2102 control signal psb_sel, which determineswhether (i) an SSU arriving at PBP 2102 via pbp_indata[31:0] will passthrough PBP 2102 and be provided to WIF 2103 normally (in non-degradedmode), or instead, (ii) PBP 2102 will generate and output accumulatedparity information to WIF 2103 (in degraded mode), rather than thearriving SSU. Handshaking between RDE 701 and TMA 707 is implemented inTMI 2100 by RISM 2202 and WISM 2203, as described above, via signals (i)rde_tma_valid, provided by RISM 2202 to TMA 707, (ii) tma_rde_ready,provided by TMA 707 to RISM 2202, (iii) rde_tma_ready, provided by WISM2203 to TMA 707, and (iv) tma_rde_valid, provided by TMA 707 to WISM2203. Additionally, RISM 2202 provides SOH signal rde_tma_soh to TMA707, and TMA 707 provides SOH signal tma_rde_soh to WISM 2203. WIBR 2201also provides header information to WOS 2101 on 32-bit data signalwos_data[31:0] for storage in Write-Header Extraction Registers (WHER)2301.

FIG. 23 illustrates Write-Operation Sequencer (WOS) sub-block 2101 ofRDE 701. As shown, WOS 2101 includes Write-Operation State Machine(WOSM) 2300, Write-Header Extraction Registers (WHER) 2301,Write-Operation State Registers (WOSR) 2302, translator 2303,Write-Header Information Registers (WHIR) 2304, and Write-RequestConfiguration Registers (WCFR) 2305. For write requests, storage-requestframes (shown in FIG. 12) and retrieval-request frames (shown in FIG.13) are drawn into WIBR 2201 of TMI 2100 upon demand by WOSM 2300. It isnoted that the information stored in many of these registers changesquickly, i.e., as each SSU is written to disks 712.

WHER 2301 stores header information (e.g., T, RAC, starting_DSA, LENGTH,and QID) received from TMI 2100 via signal wos_data[31:0], identified bya valid SOH assertion via signal tma_rde_soh.

WOSR 2302 stores various information received from translator 2303 andmaintains various counts, including, e.g., the current DID (RAID4_DID),current DSA (DSA), current LBA (LBA), current stripe index (STRIPE),current parity rotation (PARROT), current offsets (STRIPE_SSU_OFFSET,STRIPE_DSU_OFFSET, SSU_DSU_OFFSET), current SSU count, current DSUcount, current sector count, and current dword count.

WCFR 2305 stores various information received from translator 2303,including, e.g., starting offsets (starting_STRIPE,starting_SSU_DSU_OFFSET, starting_STRIPE_DSU_OFFSET,starting_STRIPE_SSU_OFFSET, and starting_SSU_OF_STRIPE), the RAC of theoperative RAID cluster profile, transfer length LENGTH, cluster size N,chunk size K, and number of DSUs per stripe (K*(N−1)).

WHIR 2304 stores various information received from translator 2303,including, e.g., T, the starting LBA, transfer count XCNT, and currentQID.

It should be understood that not all of the foregoing information storedin the registers of WOS 2101 is used in all embodiments of the presentinvention, and that other information not specifically mentioned hereincould alternatively or additionally be stored in these registers.

Translator 2303 calculates, for each stripe being written to disks 712,the LBA corresponding to a provided DSA, using the LBA=DSA/(N−1)relationship described above. In addition to the LBA, the offsetSSU_DSU_OFFSET is obtained using the SSU_DSU_OFFSET=DSA mod (N−1)relationship described above, and the offset STRIPE_DSU_OFFSET isobtained using the STRIPE_DSU_OFFSET=DSA mod (K*(N−1)) relationshipdescribed above. The Parity-Rotation Index is also obtained, using thePARROT=STRIPE mod N relationship described above. The transfer length(LENGTH) is distributed across the RAID cluster and is adjusted for anySSU offset. When the translations have been completed, the translatedinformation (with the header information provided in the header formatsshown in FIG. 16 and FIG. 17) is loaded into registers WHIR 2304 andWCFR 2305.

For storage requests, WOSM 2300 initializes the state registers of WOSR2302 with state information derived from the starting DSA at whichstorage is to begin (starting_DSA). Header and configuration informationis pulled through PBP 2102 into Pending-Write Request FIFO (WPF) 2603 ofWIF 2103, for all drives in the referenced RAID-Array Cluster. WOS 2101maintains a dword count, and WISM 2203 of TMI 2100 advances throughsectors as stripes are completed, under control by WOSM 2300.

For retrieval requests, there is no data associated in the write path.However, header and configuration information is pulled through PBP2102, for all drives in the referenced RAID-Array Cluster, into WPF 2603of WIF 2103 and into IRF 2700 of ROS 2104.

Header information from the contents of register WHER 2301 (T, RAC,starting_DSA, LENGTH, and QID) is provided via data signalirf_data[64:0] to IRF 2700 of ROS 2104.

During degraded-mode operation, storage and retrieval requests targetedfor the degraded drive are not entered into WPF 2603 of WIF 2103.Logical-Drive Identifier RAID4_DID_ldeg of the degraded drive is derivedfrom the value stored in the ldeg[9:7] bits of the operative VAP storedin one of RAC Profile Registers 0-15 (rRAC0-rRAC15, discussed in furtherdetail with respect to Tables 21 and 22 below), and the PARROT from WCFR2305. For the write path, all writes to WPF 2603 of WIF 2103 areinhibited when (i) the value stored in the degraded[6] bit of thecorresponding VAP stored in one of rRAC0-rRAC 1 5 is TRUE, and (ii) thevalue read from ldeg[9:7] of the corresponding VAP stored in one ofrRAC0-rRAC15 matches the RAID5_DID obtained by presenting the RAID4_DIDto PARROT DID Map 2601 of WIF 2103.

If the tail end of a storage request, as determined by the LENGTH plusSSU_DSU_OFFSET intersects an SSU, the remaining sectors of the SSU arewritten with zeros.

WIF 2103 reads various values out of the registers of WOS 2101,including the following. The current PARROT (PARROT[2:0]) andRAID4_DID[3:0] are read from WOSR 2302, the indicator for the operativeRAID-Array Cluster RAC[3:0] is read from WCFR 2305, and headerinformation T, LBA[47:0], XCNT[12:0], and QID[6:0] are read from WHIR2304.

With reference to the state diagram of FIG. 24, the operation of WOSM2300 will now be described.

The Write-Idle (WIDLE) state is the initial idle or ready-resting statewhile waiting for an SOH to be asserted, at which point WOSM 2300transitions to the Write-Translate (WTRAN) state. In the WTRAN state,header information extracted from a request header received from TMA 707is copied, manipulated, and translated to initialize the WHER 2301, WOSR2302, WCFR 2305, and WHIR 2304 register sets, and an entry is written toIRF 2700 of ROS 2104, after which WOSM 2300 transitions to the WriteHeader-Information Requests (WHIRs) state.

In the WHIRs state, translated header information is written to WPF 2603of WIF 2103, for each drive of the operative RAID-Array Cluster Profile,after which WOSM 2300 transitions to the Write DSUs (WDSUs) state. Iffield T has a value of 1, i.e., a retrieval operation is taking place,then WOSM 2300 transitions to the WIDLE state to wait for another SOH.

In the WDSUs state, DSUs are presented in arrival sequence(RAID4_DID<N−1) to WPF 2603 of WIF 2103, for each drive of the operativeRAID-Array Cluster Profile. If the current DSU count is greater thanLENGTH, then WOSM 2300 transitions to the Write Padded Sectors (WPADs)state, and if the drive identification count reaches N−1, then WOSM 2300transitions to the Write PSU (WPSU) state.

In the WPADs state, zero-padded sectors are presented sequentially(RAID4_DID<N−1) to WPF 2603 for each drive of the operative RAID-ArrayCluster Profile. If the current drive identifier reaches N−1, then WOSM2300 transitions to the WPSU state.

In the WPSU state, the PSU (RAID4_DID==N−1) is presented to WPF 2603. Ifthe current SSU count is less than XCNT, then WOSM 2300 transitions tothe WDSUs state, and if the current SSU count reaches XCNT, then WOSM2300 transitions to the WIDLE state.

In the WDSUs, WPADs, and WPSU states, sectors destined for degradeddrives (i.e., when (i) the value stored in the degraded[6] bit of thecorresponding profile stored in one of rRAC0-rRAC15 is TRUE, and (ii)the value read from ldeg[9:7] of the corresponding VAP stored in one ofrRAC0-rRAC15 matches the RAID5_DID obtained by presenting the RAID4_DIDto PARROT DID Map 2601 of WIF 2103) are blanked, i.e., these sectors arenot loaded into WPF 2603.

FIG. 25 illustrates Parity-Block Processor (PBP) sub-block 2102 of RDE701. PBP 2102 performs block-parity generation on SSU sector datareceived from WIBR 2201 of TMI 2100, as directed by WOSM 2300 of WOS2101. Instead of storing parity information for each SSU sector (as intraditional RAID array systems), which would require substantialoverhead in terms of memory and time, PBP 2102 accumulates parityinformation in a single buffer, i.e., Parity-Sector Buffer (PSB) 2500(e.g., a 128×32-bit RAM with separate read and write ports). As thefirst sector of an SSU flows to WIF 2103, this sector is also copied toPSB 2500. As subsequent sectors flow through to WIF 2103, the contentsof PSB 2500 are replaced with the XOR of (i) its previous contents and(ii) the arriving SSU sector data, thereby accumulating parityinformation in PSB 2500. When N−1 sector units have been transferred,PSB 2500 is transferred and cleared. Signal psb_sel received from WIBR2201 of TMI 2100 controls multiplexer 2501, thereby determining whether(i) an SSU arriving at PBP 2102 via pbp_indata[31:0] will pass throughPBP 2102 and be provided as pbp_outdata[31:0] to WIF 2103 normally (innon-degraded mode), or instead, (ii) PBP 2102 will generate and outputas pbp_outdata[31:0] accumulated parity information to WIF 2103 (indegraded mode), rather than the arriving SSU.

FIG. 26 illustrates Write-Interface (WIF) sub-block 2103 of RDE 701. WIF2103 includes Write-Header Information-Buffer Register (WHIBR) 2600,PARROT DID Map 2601, PHYS DID Map 2602, Pending-Write Request FIFO (WPF)2603, and state machine 2604. WIF 2103 buffers requests for storage andretrieval operations and communicates those requests to MDC 705. Writeoperations are executed as commanded by WOS 2101 and, as these requestsare written to WPF 2603 (e.g., a 2 k×36-bit FIFO) and then sent to MDC705, information is also written by WOS 2101 to IRF 2700 of ROS 2104.WHIBR 2600 holds header information to be multiplexed with storagerequest data. Accordingly, WHIBR 2600 receives from WHIR 2304 of WOS2101 header information including LBA[47:0], XCNT[12:0], QID[6:0], andT. This header information (as shown in FIG. 16 and FIG. 17) is writtenfor each drive in the RAID-Array Cluster once per storage or retrievalrequest.

WIF 2103 performs RAC mapping from Logical-Drive Identifier toPhysical-Drive Identifier upon demand by WOS 2101. Accordingly, PARROTDID Map 2601 receives the stripe's Parity-Rotation Index PARROT[2:0] andLogical-Drive Identifier RAID4_DID[3:0] from WOSR 2302 of WOS 2101, andPHYS DID Map 2602 receives the selected RAID-Array Cluster registersRAC[3:0] from WCFR 2305 of WOS 2101. Parity rotation is accomplished bysimply using PARROT DID Map 2601 to map RAID4_DID[3:0] to theparity-rotated Logical-Drive Identifier (RAID5_DID) using PARROT[2:0].PHYS DID Map 2602 handles mapping of a parity-rotated logical disk arraydrive number (RAID5_DID) to a physical drive number (PDID), which isperformed using the operative VAP stored in one of RAC Profile Registers0-15 (rRACMAP0-rRACMAP15, discussed in further detail with respect toTables 23 and 24 below) identified by RAC[3:0]. The drive identifierpdid[2:0], along with header information from WHIBR 2600 multiplexedwith storage request data received from WIBR 2201 of TMI 2100 via PBP2102 are provided to WPF 2603 for storage. WPF 2603 provides to MDC 705(i) the current drive identifier via signal rde_mdc_wdid[2:0], (ii) themultiplexed header and data via signal rde_mdc_data[31:0], and (iii) SOHsignal rde_mdc_soh. State machine 2604 unloads WPF 2603 and executes awrite-interface handshake with MDC 705 via the mdc_rde_ready[7:0] signaland the rde_mdc_valid signal. The rde_mdc_valid signal is deassertedwhen WPF 2603 has been emptied, when a new physical DID is to bepresented via signal rde_mdc_wdid[2:0], or when the sampled signalmdc_rde_ready[7:0] indicates that MDC 705 is not ready.

FIG. 27 illustrates the Read-Operation Sequencer (ROS) sub-block 2104 ofRDE 701. ROS 2104 includes Issued-Request FIFO (IRF) 2700,Read-Operation State Registers (ROSR) 2701, Read-Response ConfigurationRegisters (RCFR) 2702, Request-Information Response Registers (RIRR)2703, translator 2704, Response-Header Information Register (RHIR) 2705,Response-Header Error Register (RHER) 2706, and Read-Operation StateMachine (ROSM) 2707. IRF 2700 (e.g., a 64×64-bit FIFO) receives headerinformation (as shown in FIG. 28, described below) via data signalirf_data[64:0] from WHER 2301 of WOS 2101, which ROS 2104 uses tomonitor and confirm responses to issued requests. It is noted that theinformation stored in many of these registers changes quickly, i.e., aseach SSU is being read from disks 712.

RIRR 2703 stores header information (e.g., T, RAC, starting_DSA, LENGTH,and QID) received from IRF 2700. Registers ROSR 2701 and RCFR 2702 areinitialized from this header information.

ROSR 2701 stores various information received from translator 2704 andmaintains various counts, including, e.g., the current DID(current_did), current DSA (DSA), current LBA (LBA), current stripeindex (STRIPE), current parity rotation (PARROT), current sector count,and current dword count.

RCFR 2702 stores various information received from translator 2704,including, e.g., offsets (SSU_DSU_OFFSET, STRIPE_DSU_OFFSET, andSTRIPE_SSU_OFFSET), T, the operative RAID cluster profile RAC, QID,transfer length LENGTH, transfer count XCNT, starting LBA, degradedDDID, cluster size N, chunk size K, and number of DSUs per stripe(K*(N−1)).

RHIR 2705 stores various information read out of RIRR 2703, including,e.g., T and the current QID.

RHER 2706 stores various retrieval-response information received fromRIF 2105 on rif_data[31:0], including T, the current QID, and error bitE.

It should be understood that not all of the foregoing information storedin the registers of ROS 2104 is used in all embodiments of the presentinvention, and that other information not specifically mentioned hereincould alternatively or additionally stored in these registers.

Translator 2704 reads out of RIRR 2703 header information (i.e., T, RAC,starting_DSA, LENGTH, and QID, as stored in register WHER 2301) toperform, for each stripe being read from disks 712, substantially thesame calculations (described above) that translator 2303 performs forDSA translations in the write path using WHER 2301 of WOS 2101. When thetranslations have been completed, the translated information is loadedinto registers ROSR 2701 and RCFR 2702. Register RIRR 2703 also suppliesthe T and QID fields to register RHIR 2705, which provides thisinformation to the Response-Header Information-Buffer Register block(RHIBR) of BPR 2106 via signals rhir_t and rhir_qid[6:0], respectively.Register RIER receives the T, E, and QID fields from the rif_data[31:0]stream provided by RIF 2105 and provides this information to ROSM 2707in detecting response errors from MDC 705. ROSM 2707 receives signalbpr_parity_check from BPR 2106 and signal rif_soh from RIF 2105, whichsignals are used to generate state information, as will be described infurther detail below.

BPR 2106 reads various other values out of the registers of ROS 2104,including the DDID of a degraded drive degraded_ddid[3:0], currentdword[10:0], and DDID of the drive operative for the current transfercurrent_did[3:0] from ROSR 2701. RIF 2105 also reads various values outof the registers of ROS 2104, including current_did[3:0], PARROT(PARROT[2:0]), and the indicator for the operative RAID-Array ClusterRAC[3:0], all of which are read from RCFR 2702.

ROS 2104 performs error handling as follows. If a retrieval-responseerror condition (stored in register E of RHER 2706) is detected, then itis marked in the Error-Status Registers (rRERR), as discussed below withrespect to Tables 1-5. If a VAP stored in one of RAC Profile Registers0-15 (rRAC0-rRAC15, discussed in further detail with respect to Tables21 and 22 below) indicates that MDC 705 has detected an error due to adegraded volume, then ROS 2104 “back-annotates” the operative RAID-ArrayRegister profile (specified by wcfr_rac[3:0]) with the RAID5_DIDcorresponding to the degraded drive, the degraded[6] bit in registerRCFR 2702 is set to indicate a degraded drive, and the check bit of thedegraded register in the corresponding VAP stored in rRAC0-rRAC15 iscleared. Such back-annotation does not occur if the corresponding VAPalready indicates the drive's degraded status. If a PDID is to beaccessed by an alternate VAP for which an outstanding error-status bithas already been set in the rRERR register of the corresponding VAP(discussed in further detail below), then the RAC-profile degradedback-annotation will instead be executed for that alternate profile.

FIG. 28 is a frame-format diagram showing the format for anissued-request FIFO (IRF) frame received via signal irf_data[64:0]. Eachframe includes the following fields. At bit [63], field T indicates thetype of request, as instructed by TMA 707, and is 0 for a storagerequest (i.e., the data field contains data) and 1 for a retrievalrequest. At bits [62:56], field QID[6:0] is a queue identifiercontaining the QID for which the data is being retrieved or stored. Atbits [55:52], field RAC[3:0] indicates which RAID-Array Cluster is to beoperative for the transfer. At bits [51:36], field LENGTH[15:0]indicates the number of sectors of the contiguous length of thetransfer. At bits [35:0], field DSA[35:0] indicates the DSA of thestarting DSU to access.

With reference to the state diagram of FIG. 29, the operation of ROSM2707 will now be described. The PING states unload, into response FIFO2200 of TMI 2100, (i) the contents of the primary buffer of SSUB 3101 ofBPR 2106 and (ii) the contents of the primary Response-HeaderInformation-Buffer Register (RHIBR) 3103 of BPR 2106. Concurrently, thePONG states unload, into response FIFO 2200 of TMI 2100,

(i) the contents of the alternate buffer of SSUB 3101 of BPR 2106 and(ii) the contents of the alternate RHIBR 3103 of BPR 2106. ROSM 2707 canbe referred to as a “ping-pong state machine” because the PING statesand PONG states execute at the same time, thereby permitting concurrentuse of two different RAID-Array Clusters (e.g., rebuilding a degradedvolume on one RAC while retrieving multimedia data from a differentRAC). The PING portion of the state machine “ping-pongs” the buffers ofSSUB 3101 and RHIBR 3103, i.e., flips the primary-alternate bufferdesignations, when unloading of headers and data into response FIFO 2200of TMI 2100 is complete and the PONG portion of the state machine isready.

The PING portion of ROSM 2707 operates as follows.

The Read-Idle (RIDLE) state is the initial idle or ready-resting statewhile waiting for an IRF request header via irf_data[64:0] from WHER2301 of WOS 2101 to arrive, at which point ROSM 2707 transitions to theRead-Translate (RTRAN) state.

In the RTRAN state, header information extracted from the IRF requestheader is copied, manipulated, and translated to initialize the RHER2706, ROSR 2701, RCFR 2702, and RHIR 2705 register sets, after whichROSM 2707 transitions to the Check Response Headers (CKRHERs) state.

In the CKRHERs state, the response headers for non-degraded drives arepulled via rif_data[31:0] from RIF 2105, are matched with the issuedrequest, and are checked for errors, for each drive of the operativeRAID-Array Cluster Profile.

The Update Degraded from Check Response Headers (UPDEGCKR) state isentered from CKRHERs when the E field in a response header was set,indicating an MDC-response error (as shown in FIG. 20). The appropriateerror-status bit is set in the rRERR registers (discussed in furtherdetail below), and the operative VAP stored in one of RAC ProfileRegisters 0-15 (rRAC0-rRAC15, discussed in further detail with respectto Tables 21 and 22 below) is back-annotated, as may be appropriate (asdiscussed above).

In the Read DSUs (RDSUs) state, DSUs for non-degraded drives are pulledfrom RIF 2105 in RAID4_DID order (0<DID<N−2).

The Update Degraded from Read DSUs (UPDEGDSU) state is entered fromstate RDSUs when the E field in an unexpected response header was set,indicating an MDC-response error (as shown in FIG. 20). The appropriateerror-status bit is set in the rRERR registers (discussed in furtherdetail below), and the operative VAP stored in one of RAC ProfileRegisters 0-15 (rRAC0-rRAC15, discussed in further detail with respectto Tables 21 and 22 below) is back-annotated, as may be appropriate (asdiscussed above).

In the Read PSU (RPSU) state, the PSU for a non-degraded drive is pulledfrom RIF 2105 (RAID4_DID==(N−1)) and fed to the primary buffer of SSUB3101. Parity checking is performed and status is updated, as may benecessary, in error-status registers rERR (which are discussed infurther detail below).

The Update Degraded from Read PSU (UPDEGPSU) state is entered from stateRDPSUs when the E field in an unexpected response header was set,indicating an MDC-response error (as shown in FIG. 20). The appropriateerror-status bit is set in the rRERR registers (discussed in furtherdetail below), and the operative VAP stored in one of RAC ProfileRegisters 0-15 (rRAC0-rRAC15, discussed in further detail with respectto Tables 21 and 22 below) is back-annotated, as may be appropriate (asdiscussed above).

In the Response-Reconstruct (RREC) state, the contents of Parity-SectorBuffer (PSB) 2500 of PBP 2102 are substituted for the degraded drive(RAID4_DID==RAID4_DID_ldeg) in the primary buffer of SSUB 3101.

In the Wait-for-Pong (WT4PONG) state, the primary ping-pong buffer ofSSUB 3101 is ready, but the PING portion of ROSM 2707 is waiting for thePONG portion of ROSM 2707 to finish unloading the alternate ping-pongbuffer of SSUB 3101.

In the Ping-Pong (PINGPONG) state, the primary and alternate ping-pongbuffers of SSUB 3101 and RHIBR 3103 are ping-ponged, i.e., the primaryand alternate buffer designations are switched.

The operation of the PONG portion of ROSM 2707 operates as follows.

The Wait-for-Ping (WT4PING) state is the initial idle or ready-restingstate while waiting for PING. In this state, the PONG portion of ROSM2707 is ready to feed the next alternate buffer contents of SSUB 3101 toresponse FIFO 2200 of TMI 2100. In other words, the PONG portion of ROSM2707 is ready.

In the Transfer-Response Header-Information Buffer Register (TRHIR)state, a “dirty” (used) alternate RHIBR has been ping-ponged and ispresented to response FIFO 2200 of TMI 2100. In this state, responseheaders for storage are not entered into response FIFO 2200 of TMI 2100.

In the Transfer DSUs (TDSUs) state, the alternate buffer of SSUB 3101has been ping-ponged and is presented sector-by-sector to response FIFO2200 of TMI 2100. In this state, DSUs are presented in order from 0 toN−2. Presented sector entries are only written to response FIFO 2200 ofTMI 2100 when the current DSU count is past the SSU_DSU_OFFSET and alsodoes not exceed the request LENGTH index.

FIG. 30 illustrates Read-Interface (RIF) sub-block 2105 of RDE 701. RIF2105 includes read-FIFO buffers 3000, PARROT DID Map 3001, PHYS DID Map3002, and state machine 3003. RIF 2105 retrieves and buffers responsesto issued requests described by header information from IRF 2700 of ROS2104 and provides those responses to BPR 2106. Read-FIFO buffers 3000receive from MDC 705 (i) SOH signal mdc_rde_soh and (ii) data signalmdc_rde_data[31:0]. Read-FIFO buffers 3000 receive from ROSR 2701 of ROS2104 Logical-Drive Identifier signal current_did[3:0]. Read-FIFO buffers3000 provide signal rif_data[31:0] to BPR 2106 and signal rif_soh to ROS2104. RIF 2105 performs RAC mapping from Logical-Drive Identifier toPhysical-Drive Identifier upon demand by ROS 2104. Accordingly, PARROTDID Map 3001 receives the stripe's Parity-Rotation Index PARROT[2:0] andLogical-Drive Identifier current_did[3:0] from ROSR 2701 of ROS 2104,and PHYS DID Map 3002 receives the selected RAID-Array Cluster registersRAC[3:0] from RCFR 2702 of ROS 2104. Parity rotation is accomplished bysimply using PARROT DID Map 3001 to map current_did[3:0] to theparity-rotated Logical-Drive Identifier (RAID5_DID) using PARROT[2:0].PHYS DID Map 2602 handles mapping of the (RAID5_DID) parity-rotatedlogical disk array drive numbers to physical drive numbers (PDID), whichis performed using the VAP stored in one of RAC Profile Registers 0-15(rRACMAP0-rRACMAP15, discussed in further detail with respect to Tables23 and 24 below) identified by RAC[3:0]. The drive identifierrde_mdc_pdid[2:0] is then supplied to MDC 705. State machine 3003receives signals mdc_rde_rdid[2:0], mdc_rde_valid, and mdc_rde_soh fromMDC 705 and executes a read-interface handshake with MDC 705 via therde_mdc_ready signal and the mdc_rde_valid signal. The rde_mdc_readysignal is asserted when read-FIFO buffers 3000 have been emptied toindicate that RDE 701 is ready to receive headers and data from MDC 705on data signal mdc_rde_data[31:0].

FIG. 31 illustrates Block-Parity Reconstructor (BPR) sub-block 2106 ofRDE 701. BPR 2106 passes retrieved data to TMI 2100 and reconstructsdata when operating in degraded mode. BPR 2106 includes RetrievalParity-Sector Buffer (RPSB) 3100, Stripe Sector-Unit Buffer (SSUB) 3101,Sector Sequencer (SSEQ) 3102, and Response-Header Information-BufferRegister (RHIBR) 3103. BPR 2106 receives signal rpsb_sel, whichindicates a degraded volume, from the degraded[6] bit of the operativeVAP stored in one of RAC Profile Registers 0-15 (rRAC0-rRAC15, discussedin further detail with respect to Tables 21 and 22 below). BPR 2106receives data signal rif_data[31:0] from RIF 2105. BPR 2106 receivesheader information from ROS 2104 via signals degraded_ddid[3:0],current_did[3:0], and dword[10:0]. BPR 2106 receives the T and QIDfields from ROS 2104 via signals rhir_t and rhir_qid[6:0], respectively.BPR 2106 provides control signal bpr_parity_check to ROS 2104 and datasignal bpr_data[32:0] to TMI 2100. The operation of BPR 2106 is directedby ROS 2104. SSUB 3101 (e.g., 2×1k×32-bit single-port RAMs) is a dualping-pong buffer (or “double buffer”). A ping-pong buffer contains apair of storage arrays (a “primary buffer” and an “alternate buffer”).Data received into a ping-pong buffer from a first bus is written into afirst array, while data is read out of the second array and supplied toa second bus. The read and write functions of the two storage arrays areinterchanged back and forth (“ping-ponged”) from time to time, so thatdata is alternatingly written into the first array and then the secondarray, and data is alternatingly read out from the second array and thenthe first array, in an opposite manner from that used for the writingoperation. Accordingly, SSUB 3101 contains a primary buffer and analternate buffer, which are alternatingly used to build SSUs. RetrievedSSUs flow through RPSB 3100 (e.g., a 128×32-bit RAM with separate readand write ports) and become logically organized in SSUB 3101, to bestored into one of the two buffers of SSUB 3101, as selected throughSSEQ 3102.

RPSB 3100 is similar to PSB 2500 of PBP 2102 because, as these retrievedsectors flow through RPSB 3100, XOR calculations are accumulated in RPSB3100. Signal rpsb_sel controls multiplexer 2501, determining whether anSSU arriving at RPSB 3100 will (i) cause the SSU to pass through RPSB3100 and be provided to SSUB 3101 normally (in non-degraded mode), orinstead, (ii) cause RPSB 3100 to generate and output accumulated parityinformation to SSUB 3101 (in degraded mode).

In degraded mode, the reconstructed sector corresponding to the faileddrive is loaded from the parity information accumulated in RPSB 3100.(In non-degraded mode, the contents of RPSB 3100 should be zero.) Theparity information generated by RPSB 3100 is therefore a logical OR ofthe contents of RPSB 3100, i.e., a sequentially-accumulated logical ORof XOR results written to RPSB 3100. If this parity information is notzero, then error bit E is set in RHER 2706 of ROS 2104.

Either the primary buffer or the alternate buffer of SSUB 3101 is usedto build an SSU. When the SSU is complete, SSUB 3101 is ping-ponged sothat the other buffer of SSUB 3101 is selected to build the next SSU. Asthe next SSU is built, the previously-completed SSU is fed in logicalorder to response FIFO 2200 of TMI 2100.

RHIBR 3103 is a dual ping-pong buffer containing a primary buffer and analternate buffer that are ping-ponged in tandem with the buffer of SSUB3101, and RHIBR 3103 holds header information to be multiplexed with SSUdata from the corresponding primary or alternate buffer of SSUB 3101.Accordingly, the header information received via signals rhir_t andrhir_qid[6:0] from register RIRR 2703 of ROS 2104, multiplexed with SSUdata from SSUB 3101, is provided to response FIFO 2200 of TMI 2100 on33-bit data signal bpr_data[32:0] and is only written into response FIFO2200 of TMI 2100 at the beginning of a response frame, i.e., when theQID changes.

FIG. 32 is a block diagram of AAP-Interface (AAI) sub-block 2107 andControl/Status Registers (CSR) 2108 of RDE 701. As fully discussed abovewith reference to FIG. 10, AAI 2107 receives from AAP 702 signalscore_clk, reset_ccn, aap_hwdatad[31:0], aap_haddrd[27:0], aap_rde_hseld,aap_hwrited, and aap_htransd[1:0] and provides to AAP 702 signalsrde_aap_hrdatad[31:0], rde_aap_hreadyd, rde_aap_hrespd, rde_aap_inth,and rde_aap_intl. AAI 2107 exchanges data and control signals with CSR2108, which stores memory-mapped processor-accessible registers andmemories that are used by the various sub-blocks of RDE 701.

CSR Registers

CSR 2108 (e.g., a 32×32-bit memory) includes three categories ofregisters: (i) Error-Status Registers (rRERR), (ii) RAC-ProfileRegisters (rRAC), and (iii) an RDE-Control Register (rRCTL).

In the Error-Status Registers (rRERR), error-status bits are set to theasserted state by RDE 701 when errors are recognized. Each of registersrRERR has a corresponding high-priority interrupt-mask register and acorresponding low-priority interrupt-mask register. The high-priorityinterrupt request is asserted when any error-status bit and thecorresponding high-priority interrupt mask bit are both asserted.Similarly, the low-priority interrupt request is asserted when anyerror-status bit and the corresponding low-priority interrupt mask bitare both asserted. In the event error-status bit E for a given drive hasa value of 1, or if there is T mismatch (e.g., IRF 2700 contains T=0,but MDC 705 contains T−1) or a QID mismatch, both the rRERR registersand the appropriate rRAC registers will be updated, so that the drivewith the error is marked as degraded. For a parity error detected in BPR2106 after ROSM 2707 leaves the RPSU state, i.e., accumulated ORoperations on the XOR bits result in a value of 1, the RQIDPE registersare updated, depending on the QID value, but no drives are marked asdegraded, and no other registers are updated (assuming that the checkbit is set in the operative RAC-profile register).

The following register map Table 1 shows the rERR registers that arebitmapped per DID register, for errors received from MDC 705 that occurduring either actual or attempted read or write operations on disks 712via MDC 705.

TABLE 1 rRERR-Response Errors Bitmapped per DID Register Register BitName Offset Position Field Class Description rRERR 0xB0000010 31:24Reserved R Reserved. Always zero 23:16 MISME{7<DID<0} R mismatch Errorbit mapped per 7 < PDID <0 (Expected response ≠ MDC Response) Defaultsto zero. 15:8  MDCRE{7<DID<0} R MDC Retrieval Error bit mapped per 7 <PDID <0 (MDC Response Error marked with T ==1) Defaults to zero. 7:0MDCSE{7<DID<0} R MDC Storage Error bit mapped per 7 < PDID <0 (MDCResponse Error marked with T ==0) Defaults to zero.

The following register map Tables 2-5 show the rRERR registers thatstore bits indicating retrieval-response errors, bitmapped per QID.

TABLE 2 rQIDPE3 Queue-Identified Parity-Error Bitmapped Register 3Register Bit Name Offset Position Field Class Description rQIDPE30xB0000020 31:0 PE{127<QID<96} R Parity Error bit mapped per 127 < QID<96 Defaults to zero.

TABLE 3 rQIDPE2 Queue-Identified Parity-Error Bitmapped Register 2Register Bit Name Offset Position Field Class Description rQIDPE20xB0000028 31:0 PE{95<QID<64} R Parity Error bit mapped per 95 < QID <64Defaults to zero.

TABLE 4 rQIDPE1 Queue-Identified Parity-Error Bitmapped Register 1Register Bit Name Offset Position Field Class Description rQIDPE10xB0000030 31:0 PE{63<QID<32} R Parity Error bit mapped per 63 < QID <32Defaults to zero.

TABLE 5 rQIDPE0 Queue-Identified Parity-Error Bitmapped Register 0Register Bit Name Offset Postion Field Class Description rQIDPE00xB0000038 31:0 PE{31<QID<0} R Parity Error bit mapped per 31 < QID <0Defaults to zero.

The following register map Tables 6-10 show the rRERR high-priorityinterrupt-mask registers. When the bits of these registers are set to 1,high-priority interrupt requests are generated when the correspondingbits in the rRERR register are set.

TABLE 6 rRERR-Response Errors Bitmapped per DID High-PriorityInterrupt-Mask Register Register Bit Name Offset Position Field ClassDescription rRERRH 0xB0000040 31:24 Reserved RW Reserved. Defaults tozero 23:0  HIM RW These bits are set to 1 to enable high priority in-terrupts to be generated when the corresponding bits in the rRERRregister are set Defaults to zero.

TABLE 7 rQIDPE3 Queue-Identified Parity-Error Bitmapped High-PriorityInterrupt-Mask Register 3 Register Bit Name Offset Position Field ClassDescription rQIDPE3H 0xB0000050 31:0 HIM RW These bits are set to 1 toenable high priority interrupts to be generated when the correspondingbits in the rRQIDPE3 register are set Defaults to zero.

TABLE 8 rQIDPE2 Queue-Identified Parity-Error Bitmapped High-PriorityInterrupt-Mask Register 2 Register Bit Name Offset Position Field ClassDescription rQIDPE2H 0xB0000058 31:0 HIM RW These bits are set to 1 toenable high priority interrupts to be generated when the correspondingbits in the rRQIDPE2 register are set Defaults to zero.

TABLE 9 rQIDPE1 Queue-Identified Parity-Error Bitmapped High-PriorityInterrupt-Mask Register 1 Register Bit Name Offset Position Field ClassDescription rQIDPE1H 0xB0000060 31:0 HIM RW These bits are set to 1 toenable high priority interrupts to be generated when the correspondingbits in the rRQIDPE1 register are set Defaults to zero.

TABLE 10 rQIDPE0 Queue-Identified Parity-Error Bitmapped High-PriorityInterrupt-Mask Register 0 Register Bit Name Offset Position Field ClassDescription rQIDPE0H 0xB0000068 31:0 HIM RW These bits are set to 1 toenable high priority interrupts to be generated when the correspondingbits in the rRQIDPE0 register are set Defaults to zero.

The following register map Tables 11-15 show the rRERR low-priorityinterrupt-mask registers. When the bits of these registers are set to 1,low-priority interrupt requests are generated when the correspondingbits in the rRERR register are set.

TABLE 11 rRERR-Response Errors Bitmapped per DID Low-PriorityInterrupt-Mask Register Register Bit Name Offset Position Field ClassDescription rRERRL 0xB0000048 31:24 Reserved RW Reserved. Defaults tozero 23:0  LIM RW These bits are set to 1 to enable low priorityinterrupts to be generated when the cor- responding bits in the rRERRregister are set Defaults to zero.

TABLE 12 rQIDPE3 Queue-Identified Parity-Error Bitmapped Low-PriorityInterrupt-Mask Register 3 Register Bit Name Offset Position Field ClassDescription rQIDPE3L 0xB0000070 31:0 LIM RW These bits are set to 1 toenable low priority interrupts to be generated when the correspondingbits in the rRQIDPE3 register are set Defaults to zero.

TABLE 13 rQIDPE2 Queue-Identified Parity-Error Bitmapped Low-PriorityInterrupt-Mask Register 2 Register Bit Name Offset Position Field ClassDescription rQIDPE2L 0xB0000078 31:0 LIM RW These bits are set to 1 toenable low priority interrupts to be generated when the correspondingbits in the rRQIDPE2 register are set Defaults to zero.

TABLE 14 rQIDPE1 Queue-Identified Parity-Error Bitmapped Low-PriorityInterrupt-Mask Register 1 Register Bit Name Offset Position Field ClassDescription rQIDPE1L 0xB0000080 31:0 LIM RW These bits are set to 1 toenable low priority interrupts to be generated when the correspondingbits in the rRQIDPE1 register are set Defaults to zero.

TABLE 15 Queue-Identified Parity-Error Bitmapped Low-PriorityInterrupt-Mask Register 0 Register Bit Name Offset Position Field ClassDescription rQIDPE0L 0xB0000088 31:0 LIM RW These bits are set to 1 toenable low priority interrupts to be generated when the correspondingbits in the rRQIDPE0 register are set Defaults to zero.

Each of error-status registers rRERR has a corresponding clear register.The error-status bits are cleared (returned to the deasserted state)when the corresponding bits are asserted in a write operation to theclear register. When all of the masked error-status bits have beencleared, the corresponding interrupt request is deasserted. Thefollowing register map Tables 16-20 show the clear registers.

TABLE 16 rRQIDPE3 Queue-Identified Parity-Error Bitmapped Clear Register3 Register Bit Name Offset Position Field Class Description rQIDPE3C0xB0000090 31:0 Clear W These bits are set to 1 to clear the corre-sponding bits in the rRQIDPE3 register Defaults to zero.

TABLE 17 rQIDPE2 Queue-Identified Parity-Error Bitmapped Clear Register2 Register Bit Name Offset Position Field Class Description rQIDPE2C0xB0000098 31:0 Clear W These bits are set to 1 to clear the corre-sponding bits in the rRQIDPE2 register Defaults to zero.

TABLE 18 rRQIDPE1 Queue-Identified Parity-Error Bitmapped Clear Register1 Register Bit Name Offset Position Field Class Description rQIDPE1C0xB00000A0 31:0 Clear W These bits are set to 1 to clear the corre-sponding bits in the rRQIDPE1 register Defaults to zero.

TABLE 19 rQIDPE0 Queue-Identified Parity-Error Bitmapped Clear Register0 Register Bit Name Offset Position Field Class Description rQIDPE0C0xB00000A8 31:0 Clear W These bits are set to 1 to clear the corre-sponding bits in the rRQIDPE0 register Defaults to zero.

TABLE 20 rRERR-Response Errors Bitmapped per DID-Register Clear RegisterBit Name Offset Position Field Class Description rRERRC 0xB00000B0 31:24Reserved NA 23:0  Clear W These bits are set to 1 to clear thecorresponding bits in the rRERR register Defaults to zero.

In the unlikely event that, when an error-status bit is being cleared, acorresponding error event for the same error-status bit has been newlyrecognized on the same clock cycle, that bit should remain cleared. Inother words, the software-mandated clearing operation directed to AAP702 has a higher priority than the setting of the repeat event.Presumably, software has taken notice of the initial event. If, however,the newly-recognized event arrives at the error-status register one ormore clock cycles before or after that error-status register is cleared,then there is no collision, and there is no obstructing refractoryinterval.

The RAC-Profile Registers (rRAC) store information about each of theVAPs, including chunk size (K), number of DSUs per stripe (K*(N−1)),whether parity-checking is enabled, logical number of a degraded drive(RAID_(—)5_DID_ldeg), cluster degraded status, RAID level, cluster size,and physical-to-logical drive mappings. In the write path, the operativeRAID-Array Cluster Profile is chosen as indexed by the request'sRAC[3:0] field (as shown in FIG. 12). In the read path, the operativeRAID-Array Cluster Profile is chosen as indexed by the response'sRAC[3:0] field (as shown in FIG. 13). In the unlikely occurrence that anerror-induced back-annotation event is recognized simultaneously with aprocessor-mandated update on an operative VAP, the software-mandatedupdate directed by AAP 702 overrides the back-annotation, because theupdate is based on the “stale” profile. If, however, thenewly-recognized event arrives at the profile register one or more clockcycles before or after the register is cleared, then there is nocollision, and there is no obstructing refractory interval. Suchcollisions could be avoided by software restricting configurationupdates to “spare” out-of-service profiles that will not be operativefor outstanding requests, and then switching to the spare updatedalternate profile. It is noted that, for each RAID-Array Cluster, a RAIDlevel is stored in bits[5:4], which can be either RAID-5, RAID-4,RAID-0, or “Just a Bunch of Disks” (JBOD). Whereas a RAID system storesthe same data redundantly on multiple physical disks that neverthelessappear to the operating system as a single disk, JBOD also makes thephysical disks appear to be a single one, but accomplishes this bycombining the drives into one larger logical drive. Accordingly, JBODhas no advantages over using separate disks independently and providesnone of the fault tolerance or performance benefits of RAID.Nevertheless, JBOD may be useful for certain applications, and an RACcluster can utilize a JBOD scheme instead of a RAID scheme, if a user sodesires. The following register map Tables 21-24 show the RAC Profileregisters.

TABLE 21 rRAC0 RAID-Array Cluster Register Register Bit Name OffsetPosition Field Class Description rRAC0 0xB0000100 31:23 Chunk-size RWNumber of sectors per chunk (K) Defaults to zero. 22:12 Stripe DSUs RWNumber of Data Sectors per stripe K*(N − 1) Defaults to zero. 11Reserved RW 10 Check RW Enable Parity Checking Defaults to zero. 9:7Ldeg RW Logical Number (RAID5_DID) of the degraded drive Defaults tozero. 6 degraded RW this Cluster to be treated as degraded Defaults tozero. 5:4 RAID level RW 0 = RAID level 5 (left symmetric parityplacement) 1 = RAID level 4 (parity without rotation) 2 = RAID level 0(stripeing, no parity) 3 = JBOD Just a Bunch of Disks - cluster sizemust be one Defaults to zero. 3:0 Cluster-size RW (N) Number of drivesconfigured for cluster Defaults to zero.

TABLE 22 rRAC1-15 RAID-Array Cluster Registers 1-15 Register Bit NameOffset Position Field Class Description rRAC1-15 0x80000100 + 31:0 AllRW same format as 8*RAC[3:0] rRAC0. Defaults to zero.

TABLE 23 rRACMAP0 Drive-Mapping Register for RAID-Array Cluster 0Register Bit Name Offset Position Field Class Description rRACMAP00xB0000180 31 Reserved NA Reserved. Always zero 30:28 PD7 RW PhysicalDrive mapped to Logical Drive 7 Defaults to zero. 27 Reserved NAReserved. Always zero 26:24 PD6 RW Physical Drive mapped to LogicalDrive 6 Defaults to zero. 23 Reserved NA Reserved. Always zero 22:20 PD5RW Physical Drive mapped to Logical Drive 5 Defaults to zero. 19Reserved NA Reserved. Always zero 18:16 PD4 RW Physical Drive mapped toLogical Drive 4 Defaults to zero. 15 Reserved NA Reserved. Always zero14:12 PD3 RW Physical Drive mapped to Logical Drive 3 Defaults to zero.11 Reserved NA Reserved. Always zero 10:8  PD2 RW Physical Drive mappedto Logical Drive 2 Defaults to zero.  7 Reserved NA Reserved. Alwayszero  6:4 PD1 RW Physical Drive mapped to Logical Drive 1 Defaults tozero.  3 Reserved NA Reserved. Always zero  2:0 PD0 RW Physical Drivemapped to Logical Drive 0 Defaults to zero.

TABLE 24 rRACMAP1-15 Drive-Mapping Registers for RAID-Array Clusters1-15 Register Bit Name Offset Position Field Class DescriptionrRACMAP1-15 0xB0000180 + 31:0 All RW same format as 8*RAC[3:0] rRACMAP0.Defaults to zero

The RDE-Control Register (rRCTL) controls enabling of the interfacesbetween RDE 701 (as shown in FIG. 10) and the various other blocks ofsystem 700, as well as the zeroing of the various FIFOs and associatedpipelines in RDE 701. If the Enable TMA-RDE Interface (ETMARDE) bit isset, then the TMA-RDE interface is enabled and, if not set, then signalrde_tma_ready will not be asserted. If the Enable RDE-TMA Interface(ERDETMA) bit is set, then the RDE-TMA interface is enabled and, if notset, then signal tma_rde_ready will be perceived to be deasserted, andsignal rde_tma_valid will not be asserted. If the Enable MDC-RDEInterface (EMDCRDE) bit is set, then the MDC-RDE interface is enabledand, if not set, then signal rde_mdc_ready will not be asserted. If theEnable RDE-MDC Interface (ERDEMDC) bit is set, then the RDE-MDCinterface is enabled and, if not set, then signal mdc_rde_ready[7:0]will be perceived to be deasserted, and signal rde_mdc_valid will not beasserted. If the Zero-Response Pipeline (ZRSP) bit is set, then theentire response pipeline of RDE 701, including response FIFO 2200 of TMI2100, and Read-Operation State Machine (ROSM) 2707 of ROS 2104 iscleared. If the Zero-Issued Request FIFO (ZIRF) bit is set, thenissued-request FIFO (IRF) 2700 of ROS 2104 is cleared. If theZero-Request Pipeline (ZREQ) bit is set, then the entire requestpipeline of RDE 701, including FIFO (WPF) 2603 of WIF 2103, andWrite-Operation State Machine (WOSM) of WOS 2101 is cleared. Thefollowing register map Table 25 shows the RDE Control Register.

TABLE 25 rRCTL RDE-Control Register Register Bit Name Address PositionField Class Description rRCTL 31:7 Reserved RW Reserved. 0xB0000000Defaults to zero 6 ZREQ RW Zero Request Pipeline Defaults to zero. 5ZIRF RW Zero Issued Request FIFO Defaults to zero. 4 ZRSP RW ZeroResponse Pipeline. Defaults to zero. 3 ERDEMDC RW Enable RDE-MDCinterface. Defaults to zero. 2 EMDCRDE RW Enable MDC-RDE interface.Defaults to zero. 1 ERDETMA RW Enable RDE-TMA interface. Defaults tozero. 0 ETMARDE RW Enable TMA-RDE interface. Defaults to zero.RDE Storage Operation

The sequence of RDE 701 sending sectors to MDC 705 for write operationsproceeds by stripe from drive to drive in round-robin manner, asdiscussed above with respect to FIG. 9. As a first example, the sequenceof writing to the disk drives of a six-drive array (N=6), wherein thetransfer count is 4 (XCNT=4) and the sector sequence of0-1-2-3-4-P-5-6-7-8-9-P-10-11-12-13-14-P-15-16-17-18-19-P, would bewritten as follows. Logical disk 0 is sent a header with an XCNT of 4,followed by sectors 0, 5, 10, and 15. Thus, a header indicating XCNT=4is written to drive 0. A header with XCNT=4 is written to drive 1. Aheader with XCNT=4 is written to drive 2. A header with XCNT=4 iswritten to drive 3. A 20 header with XCNT=4 is written to drive 4. Aheader with XCNT=4 is written to drive 5. Sector 0 is written to drive0. Sector I is written to drive 1. Sector 2 is written to drive 2.Sector 3 is written to drive 3. Sector 4 is written to drive 4. Sector Pis written to drive 5. Sector 5 is written to drive 0. Sector 6 iswritten to drive 1. Sector 7 is written to drive 2. Sector 8 is writtento drive 3. Sector 9 written to drive 4. Sector P is written to drive 5.Sector 10 is written to drive 0, and so forth. Thus, RDE 701 does notrearrange data that it receives from TMA 707 but merely provides it tothe drives sector by sector, each sector being written to a successivedrive.

A second exemplary storage operation of RDE 701 will now be described,wherein the requested volume is degraded. The parameters of this storageoperation are as follows. WHER 2301 contains the following headerinformation (as shown in FIG. 14 and FIG. 15): T=0 (indicating a storagerequest), RAC=0 (profile number 0), QID=2, LENGTH=22, andstarting_DSA=28. Register rRAC contains the following VAP information:K=4 (chunk size), K*(N−1)=16 (number of data sectors per stripe), N=5(array has 5 drives), degraded=1 (degraded is TRUE), andRAID5_DID_ldeg=3 (logical number of degraded drive). WCFR 2305 containsthe following information: starting_STRIPE=1, starting_SSU_DSU_OFFSET=0,starting_STRIPE_DSU_OFFSET=12, starting_STRIPE_SSU_OFFSET=12, andstarting_SSU_OF_STRIPE=3. WOSR 2302 contains the following information:starting_PARROT=1. WHIR 2304 contains the following header information:XCNT=6, and starting_LBA=7.

(1) Header information from WHIR 2304 is provided to WPF 2603 from WHIBR2600 for each DID (RAID4_DID). As the header information is beingprovided to WPF 2603, PARROT maps the RAID4_DID to RAID5_DID. WhenDID==4 (i.e., N−1), PARROT DID Map 2601 of WIF 2103 maps to the PSU'sRAID5_DID, which matches RAID5_DID_ldeg==3, causing the information inWHIBR 2600 to be blanked. Mapping from RAID5_DID to PDID is also done byPHYS DID Map 2602 of WIF 2103 using the per-DID entries for RAC==0 inthe operative VAP stored in RAC Profile Register 0 (rRACMAP0, discussedabove with respect to Table 23).

(2) DSUs 28 through 31 flow through PBP 2102 to WPF 2603 of WIF 2103.PSU P7 is blanked because its RAID5_DID matches the RAID5_DID_ldeg==3,and so PSU P7 never enters WPF 2603.

(3) As DID wraps back around to zero, the per-SSU registers of WOSR 2302are incremented. However, in this case, the current SSU_OF_STRIPEreaches chunk size K and also wraps around. As the stripe boundary iscrossed, the current STRIPE and PARROT are incremented.

(4) DSUs 32 through 35 flow through PBP 2102 to WPF 2603 of WIF 2103.DSU 34 is blanked because its RAID5_DID matches the RAID5_DID_ldeg==3,and so DSU 34 never enters WPF 2603. This time, however, PSU P8 doesenter WPF 2603, because the current_parrot was incremented, and when itsPSU RAID4_DID (which always equals (N−1)), is mapped through PARROT DIDMap 2601 of WIF 2103, its RAID5_DID==2.

(5) Similarly, DSUs 36 through 49 and PSUs P9 through P11 either flowthrough PBP 2102 to WPF 2603 of WIF 2103 or are blanked.

(6) At DSU 50, the DSU_count matches the LENGTH, and therefore, no moredata is to be stored for QID==2 from TMI 2100, and so zero-paddingensues. Zeroes generated by PBP 2102 are passed through PSB 2500. WhenDID==N−1, P12 is provided to WPF 2603 of WIF 2103. It is noted that thezero-padding-generated DSU 50 is blanked.

(7) ROS 2104 retrieves the entry from IRF 2700, performs thecalculations in translator 2704, and updates the RIRR 2703, ROSR 2701,and RCFR 2702 registers. The response headers are read from MDC 705 andverified for each of the non-degraded drives in the RAID cluster. Whenheader checking is complete, the buffers of SSUB 3101 and RHIBR 3103 areping-ponged, and the appropriate error-status bit is updated in therRERR registers.

RDE Retrieval Operation

The sequence of operations in a first exemplary retrieval operation,where N=6, proceeds as follows.

(1) TMA 707 sends to RDE 701 a storage-request frame (as shown in FIG.12) that provides QID, RAC, starting_DSA, and LENGTH.

(2) WOS 2101 receives QID, RAC, starting_DSA, and LENGTH and calculatesa header to be generated and written to each drive. This header includesthe LBA and XCNT. In this example, the same header will be sent sixtimes, i.e., there will be six identical headers for six disks.

(3) The six headers are written to WPF 2603 of WIF 2103.

(4) WIF 2103 sends these six headers from WPF 2603 to MDC 705 in thedata control/transfer format shown in FIG. 16, and with the timing shownin FIG. 15.

(5) MDC 705 then retrieves data from the six disks, according to theinformation in the six headers that it received from RDE 701, and storesthe data into its six different FIFOs.

(6) The response data in the FIFOs of MDC 705 is transferred to RDE 701in the data control/transfer format shown in FIG. 19, and with thetiming shown in FIG. 18.

(7) Error status is updated in the rRERR registers.

A second exemplary retrieval operation of RDE 701 will now be described,wherein the requested volume is degraded. The parameters of this storageoperation are as follows. WHER 2301 contains the following headerinformation (as shown in FIG. 14 and FIG. 15): T=1 (indicating aretrieval request), RAC=0 (profile number 0), QID=3, LENGTH=12, andstarting_DSA=53. Register rRAC contains the following VAP information:K=4 (chunk size), K*(N−1)=16 (number of data sectors per stripe), N=5(array has 5 drives), degraded=1 (degraded is TRUE), andRAID5_DID_ldeg=3 (logical number of degraded drive). WCFR 2305 containsthe following information: starting_STRIPE=3, starting_SSU_DSU_OFFSET=1,starting_STRIPE_DSU_OFFSET=5, starting_STRIPE_SSU_OFFSET=4, andstarting_SSU_OF_STRIPE=1. WOSR 2302 contains the following information:starting_PARROT=3. WHIR 2304 contains the following header information:XCNT=4, and starting_LBA=13.

(1) Header information from WHIR 2304 is provided to WPF 2603 from WHIBR2600 for each DID (RAID4_DID). As the header information is beingprovided to WPF 2603, PARROT maps the RAID4_DID to RAID5_DID. WhenDID==2, PARROT DID Map 2601 of WIF 2103 maps to the RAID5_DID, whichmatches RAID5_DID_ldeg==3, causing the information in WHIBR 2600 to beblanked. Mapping from RAID5_DID to PDID is also done by PHYS DID Map2602 of WIF 2103 using the per-DID entries for RAC==0 in the operativeVAP stored in RAC Profile Register 0 (rRACMAP0, discussed above withrespect to Table 23).

(2) ROS 2104 retrieves the entry from IRF 2700, performs thecalculations in translator 2704, and updates the RIRR 2703, ROSR 2701,and RCFR 2702 registers. The response headers are read from MDC 705 andverified for each of the non-degraded drives in the RAID cluster. Whenheader checking is complete, the buffers of SSUB 3101 and RHIBR 3103 areping-ponged, and the appropriate error-status bit is updated in therRERR registers.

(3) Because this is a retrieval response, a response header is enteredinto response FIFO 2200 of TMI 2100.

(4) SSU 13 (which contains DSU 52, P13, DSU 53, and DSU 55) is retrievedfrom MDC 705, with the exception of DSU 54, whose RAID4_DID==2 maps to aRAID5_DID which matches the RAID5_DID_ldeg==3. Therefore, DSU 54 isreconstructed by BPR 2106.

(5) After another ping-pong of the buffers of SSUB 3101 and RHIBR 3103,DSU52 is not written into the TRF because thestarting_SSU_DSU_OFFSET==1. Therefore, DSU 53, reconstructed DSU 54, andDSU 55 are written into response FIFO 2200 of TMI 2100.

(6) Similarly, SSUs 14, 15, and 16 are retrieved from MDC 705 with DSU58, DSU 62, and DSU 66 reconstructed by BPR 2106, while DSU 56 throughDSU 64 are written into response FIFO 2200 of TMI 2100

(7) DSU 65 through DSU 67 are not written into response FIFO 2200 of TMI2100 because the current DSU count exceeds the LENGTH plus the drive'sSSU_DSU_OFFSET, i.e., the request was for twelve DSUs, twelve DSUs havenow been provided.

Practical RDE Applications

The use of RDE 701, consistent with certain embodiments of theinvention, enables a number of features and applications not possiblewith traditional RAID implementations, such as the following.

As discussed above, the RAC-Profile Registers (rRAC) store informationdescribing each RAID-Array Cluster and enable the implementation ofmultiple virtual-array clusters with multiple VAPs. Traditional RAIDarrays do not employ such RAC Profile Registers, but instead use either(i) fixed-hardware implementations that permit a group of drives toappear as one or (ii) software implementations that use the hostcomputer's CPU to perform RAID operations. Disadvantageously, suchtraditional hardware implementations are inflexible, and such softwareimplementations use processor and memory overhead. Moreover, neitherpermits a single set of physical drives to be used in more than oneconfiguration at a time.

According to embodiments of the present invention, two differentvirtual-array clusters can be defined for a single set of physical disksand can be used concurrently for different operations. For example, afirst virtual-array cluster could be streaming multimedia data to asoftware viewer running on computer 714, while a second virtual-arraycluster is downloading data from another source. (This is enabled by theping-pong state machine of ROSM 2707, which permits concurrent use oftwo different RAID-Array Clusters by “ping-ponging” the buffers of SSUB3101 and RHIBR 3103, discussed in further detail below.) Moreover,during a retrieval request, only two clock cycles are required for each32-bit frame (of two 32-bit quad bytes each) to be read from disks 712.A first QID can be operating on a first VAP and a second QID on a secondVAP, wherein operations for the first cluster can take placeconcurrently with operations from the second cluster. In fact, it ispossible for the two operations to be taking place on the same mediaobject by concurrently accessing two different VAPs on which the mediaobject is addressable, e.g., downloading one portion of a multimediaobject while viewing another portion of the same object.

Various media objects with different data-object file systems stored ondifferent portions of the same physical disk array can coexist and beused concurrently. The use of virtual-array profiles permits thisbecause each physical drive in the virtual array has its ownphysical-to-logical drive mapping (stored in registersrRACMAP0-rRACMAP15) and a user can therefore exploit the additionalstorage on the fourth drive by having two virtual-array clustersaccessible concurrently, each with a different file system (e.g., onewith Linux and the other with Microsoft Windows).

Remapping of logical-to-physical disk drive identifiers permitsarbitrary placement of disk drives in a set of drive bays. This ispossible because, unlike traditional RAID arrays that identify diskdrives only as physical drives (i.e., slot 0, slot 1, slot 2), multiplevirtual-array profiles can be used, and each profile can specifydifferent logical-to-physical drive assignments and drive orders, e.g.,one profile in which logical disk 0 corresponds to physical drives 1 and2, and another profile in which logical disk 1 corresponds to physicaldrive 0. This feature would permit, e.g., a new disk drive containing anupgraded operating system or other bootable partition to be added to anexisting array employing a first virtual cluster, so that a computercould be booted up from the new disk drive under a second, JBOD virtualcluster, and the new disk drive could then be used concurrently with theexisting virtual cluster.

In traditional RAID arrays, drives can be added to a RAID array onlywhen they are physically present in the array, and when drives areremoved from the array, no configuration information for these drivesare stored. The use of virtual-array profiles permits adding or droppinga drive from a virtual-array cluster, independent of whether or not thedrive is physically present in the array at the time the virtual-arrayprofile is configured.

Virtual-array profiles also permit drive partitioning andblock/chunk-sizing to be adjusted and resized on an ad-hoc basis, simplyby changing or removing one or more existing virtual-array profiles oradding one or more new virtual-array profiles containing the desiredsizing and partitioning.

Excess capacity on disk drives in a physical disk drive array can beused when integrating physical drives of varying sizes into virtualarrays, rather than all drives in the array being limited to using onlythe amount of storage available on the smallest-sized drive in thearray. For example, in a traditional RAID array containing three 40 GBdrives, if a fourth drive of 120 GB drive is added, only 40 GB of thefourth drive can be used. The use of virtual-array profiles solves thisproblem, because each physical drive in the virtual array has its ownphysical-to-logical drive mapping (stored in registersrRACMAP0-rRACMAP15) and a user can therefore exploit the additionalstorage on the fourth drive by having two virtual-array clustersaccessible concurrently: one RAID-S virtual-array cluster for the 40 GBof the first three disks and the first 40 GB of the fourth disk, and onevirtual-array cluster using a JBOD storage scheme to access theremaining 80 GB on the fourth disk. Thus, a user has the option ofbooting or updating from just a single disk drive in an array using avirtual array in a JBOD configuration, rather than always having to useall of the drives in the array, as required by traditional RAID arrays.Alternatively, the user could partition storage between profiles so thatthe first virtual-array cluster uses 35 GB of all four disks in a RAID-4configuration, and the second virtual-array cluster uses 5 GB of thefirst three disks and 85 GB of the fourth disk in a JBOD storage scheme,and so forth.

In traditional RAID arrays, a disk controller communicates directly withphysical disks in the array. Virtual-array profiles, in conjunction withthe various blocks of RDE 701 that employ buffering (and with FIFObuffers of MDC 705), permit a disk controller (e.g., MDC 705) tocommunicate to the array's physical disks via these RDE blocks, whichcontain memory and cache. Thus, when requests are made to a controllerto store data, the data can be loaded into cache memory to enhance writeperformance by reducing or eliminating delay while waiting for thephysical disk to be ready for the write operation. Moreover, usingvirtual-array clusters can interleave reads and writes for optimalperformance under a given drive-controller caching policy.

Virtual-array clusters can coexist with different feature selections onthe same physical set of drives, such as striping, mirroring, dedicatedparity, distributed parity, using just one disk drive, variable chunksizing, degraded and non-degraded disk volumes, degraded drive physicaldesignation, and/or variable cluster size.

Whereas, in traditional RAID arrays, an entire RAID array is unavailablefor reading and writing while a volume is being reconstructed,virtual-array clusters permit reconstruction of a degraded virtual arrayas a background process, while concurrently using a different virtualarray for reading and writing, without having to take all of the drivesof the array offline.

In traditional RAID arrays, during read and write operations, one entirestripe is buffered at a time and stored so that parity calculations canbe made, thereby requiring substantial processor and memory resourcesfor this cumbersome storage and calculation process. However, in certainembodiments of the present invention, as described above, parity iscalculated on-the-fly and accumulated in BPR 2106 and PBP 2102, therebyrequiring storage for only one DSU at a time in each of thesesub-blocks.

In traditional RAID arrays, when performing write operations, one sectorof data at a time is sent to various physical disks in the array, andsuch transfer of data is typically performed by software running on thehost computer, which calculates and provides addresses on these physicaldisks at which the data will be written. On the other hand, in certainembodiments of the present invention, all of these calculations are madequickly in hardware by RDE 701, thereby eliminating the need to usememory and processor resources of the host computer. Moreover, bysending one SSU at a time to MDC 705, all of the FIFO buffers of MDC 705that correspond to physical disks installed in an array are addressedconcurrently, thereby reducing or eliminating the need for substantialbuffering within the storage devices of the array, as well.

Dynamic Back-Annotation

As described above, if a retrieval-response error condition (stored inregister E of RHER 2706) is detected, then it is marked in theError-Status Registers (rRERR), and if a VAP stored in one of RACProfile Registers 0-15 (rRAC0-rRAC15) indicates that MDC 705 hasdetected an error due to a degraded volume, then ROS 2104“back-annotates” the operative RAID-Array Register profile (specified bywcfr_rac[3:0]) with the RAID5_DID corresponding to the degraded drive,the degraded[6] bit in register RCFR 2702 is set to indicate a degradeddrive, and the check bit of the degraded register in the correspondingVAP stored in rRAC0-rRAC15 is cleared. Such back-annotation permitsmissing or corrupt data stored on degraded portions of a drive to bereconstructed on-the-fly, without requiring user intervention or takingthe entire array offline. For example, if one of drives 712 fails whilea user is playing back a media object stored on drives 712, then RDE 701modifies the corresponding VAP as degraded and begins operating indegraded mode, with minimal interruption of the media-object playback tothe user.

As discussed above, the situation in which a request is made using asecond VAP to write to a drive that has been back-annotated using afirst VAP is handled by recording the DID of the degraded drive inrRERR. Thus, if MDC 705 receives the request using the second VAP, thenMDC 705 is configured to dump the request. If the second VAP is used tomake a read request to the degraded drive, then RDE 701 back-annotatesthe second VAP profile, as well. Thus, three different types ofback-annotation are possible in certain embodiments of the invention:(i) the marking and back-annotation of a single VAP (the operative VAP),(ii) propagation of the error across multiple profiles, and (iii) dumpperformed by MDC 705. For this reason, if a request is made using a VAP,regardless whether the request is a read or write request, then therequest is forwarded to both IRF 2700 and MDC 705, to prevent possiblelockup of RDE 701.

Where FIFO buffers are used in the embodiments described herein, itshould be understood that full and empty status of these buffers istaken into account, and that appropriate back-pressure is employed toprevent buffer over- and under-runs, even where such functionality isnot specifically described above.

In the embodiments described herein, for storage, TMA 707 provides onlyDSAs that are on stripe boundaries. Thus, if the transfer length is suchthat the storage operation does not complete on an SSU boundary, thenthe SSU is filled out with zero-padding. It should be understood,however, that in other embodiments of the invention, DSAs might not beprovided on stripe boundaries, in which case a scheme employingread-modify-write operations might be used, whereby an entire stripesector would be read, then replacement data would be stored in the samelocation, and then parity information would be calculated based on themodified data.

In the embodiments described herein, retrieval does not necessarilybegin and end on stripe boundaries, because it is expected that softwarewill sometimes request portions of objects, rather than entire objects.Thus, TMA 707 may request DSAs for sectors that are not on SSUboundaries. However, for degraded or parity-checking modes, the entirestarting SSU is read for reconstruction or checking. Similarly, certaintransfer lengths may result in requests not ending on SSU boundaries orstripe boundaries. It is contemplated, therefore, that RDE 701 desirablyprovides TMA 707 only with precisely the data that was requested, evenif more data is available.

While the embodiments set forth herein are described as having utilitywith RAID-4 and RAID-5 arrays, it is contemplated that support for otherRAID levels is possible. For example, RAID-1 (mirroring), could be addedfor storage by simply writing the same data to multiple drives. In thisscenario, retrieval would be implemented by specifying alternate VAPs.In fact, for storage with a RAID-Array Cluster of only two drives(N==2), RAID-4 or RAID-5 storage actually writes the same information asRAID-1, because the PSU matches the DSU in an SSU with only one DSU.

It should be understood that, although the embodiments described hereindescribe the use of two virtual-array clusters concurrently, more thantwo virtual-array clusters could alternatively be used concurrently inother embodiments of the invention.

The term “access,” as used herein with respect to logical or physicalstorage devices, refers to one or more of a read operation, a writeoperation, an erase operation, and a data reconstruction operation onthe storage device.

The broad term “storage configuration,” as used herein, should beunderstood to include RAID-0, RAID-1. RAID-10, RAID-0+1, RAID-4, andRAID-5 storage configurations, as well as JBOD, “Just One Disk Drive”(JODD), and other possible storage configurations involving one or morestorage devices in an array.

The term “resolution,” as used herein, refers to the number of sectorsthat are processed in a single storage operation. For example, aresolution of a single sector level means that information and paritystored on all of the disks in a current sector level of an array areprocessed (e.g., XOR-combined) before information and parity stored on asubsequent sector level are processed.

While the exemplary embodiments of the present invention have beendescribed with respect to processes of circuits, including possibleimplementation as a single integrated circuit, a multi-chip module, asingle card, or a multi-card circuit pack, the present invention is notso limited. As would be apparent to one skilled in the art, variousfunctions of circuit elements may also be implemented as processingblocks in a software program. Such software may be employed in, forexample, a digital signal processor, micro-controller, or generalpurpose computer.

The present invention can be embodied in the form of methods andapparatuses for practicing those methods. The present invention can alsobe embodied in the form of program code embodied in tangible media, suchas magnetic recording media, optical recording media, solid statememory, floppy diskettes, CD-ROMs, hard drives, or any othermachine-readable storage medium, wherein, when the program code isloaded into and executed by a machine, such as a computer, the machinebecomes an apparatus for practicing the invention. The present inventioncan also be embodied in the form of program code, for example, whetherstored in a storage medium, loaded into and/or executed by a machine, ortransmitted over some transmission medium or carrier, such as overelectrical wiring or cabling, through fiber optics, or viaelectromagnetic radiation, wherein, when the program code is loaded intoand executed by a machine, such as a computer, the machine becomes anapparatus for practicing the invention. When implemented on ageneral-purpose processor, the program code segments combine with theprocessor to provide a unique device that operates analogously tospecific logic circuits.

Unless explicitly stated otherwise, each numerical value and rangeshould be interpreted as being approximate as if the word “about” or“approximately” preceded the value of the value or range.

It will be further understood that various changes in the details,materials, and arrangements of the parts which have been described andillustrated in order to explain the nature of this invention may be madeby those skilled in the art without departing from the scope of theinvention as expressed in the following claims.

It should be understood that the steps of the exemplary methods setforth herein are not necessarily required to be performed in the orderdescribed, and the order of the steps of such methods should beunderstood to be merely exemplary. Likewise, additional steps may beincluded in such methods, and certain steps may be omitted or combined,in methods consistent with various embodiments of the present invention.

Although the elements in the following method claims, if any, arerecited in a particular sequence with corresponding labeling, unless theclaim recitations otherwise imply a particular sequence for implementingsome or all of those elements, those elements are not necessarilyintended to be limited to being implemented in that particular sequence.

Reference herein to “one embodiment” or “an embodiment” means that aparticular feature, structure, or characteristic described in connectionwith the embodiment can be included in at least one embodiment of theinvention. The appearances of the phrase “in one embodiment” in variousplaces in the specification are not necessarily all referring to thesame embodiment, nor are separate or alternative embodiments necessarilymutually exclusive of other embodiments. The same applies to the term“implementation.”

In a preferred embodiment of the present invention, all of the internalRDE processing is performed without using AAP 702 or any otherprocessor, and the state machines and parity calculation and generationfunctionality are all implemented using logic gates.

1. Apparatus for reading from a physical storage-device array comprisinga plurality of storage devices, wherein: the physical storage-devicearray has a plurality of sector levels, each sector level containing onesector on each storage device in the array at corresponding addresses onacross the storage devices; and the apparatus comprises: a memoryadapted to store two or more profiles, each profile defining (i) avirtual array associated with a selected set of the storage devices and(ii) one or more parameters used for accessing information from thevirtual array; a buffer (i) having a first portion and a second portionand (ii) coupled to receive data from the storage devices; and a statemachine (i) coupled to the buffer and the memory and (ii) adapted togenerate two or more successive pairs of instructions, wherein: thefirst instruction of each pair of instructions (i) is based on one ormore parameters of a first profile and (ii) is an instruction to read,into the first portion of the buffer, one sector of data from eachstorage device of a first virtual array; and the second instruction ofeach pair of instructions (i) is based on one or more parameters of asecond profile and (ii) is an instruction to read, into the secondportion of the buffer, one sector of data from each storage device of asecond virtual array.
 2. The apparatus of claim 1, wherein: in the firstinstruction, data from each sector of each storage device of the firstvirtual array is read at a first sector level; and in the secondinstruction, data from each sector of each storage device of the secondvirtual array is read at a second sector level.
 3. The apparatus ofclaim 2, wherein, for successive pairs of instructions, the first andsecond instructions are repeated at respective successive sector levelsof the first and second virtual arrays.
 4. The apparatus of claim 1,wherein the first and second profiles are associated with the same setof one or more storage devices.
 5. The apparatus of claim 4, wherein thefirst and second instructions are for reading from the same physicalstorage devices.
 6. The apparatus of claim 5, wherein the first andsecond instructions are for reading from the same object.
 7. Theapparatus of claim 1, wherein: one of the storage devices is a degradedstorage device; and the first portion of the buffer is used forreconstructing information previously stored on the degraded storagedevice.
 8. The apparatus of claim 7, wherein the second portion of thebuffer is used for streaming a media object containing at least one ofaudio and video data.
 9. The apparatus of claim 1, wherein the one orparameters indicate one or more of: the amount of data read from orwritten to the storage devices at a time, whether chunks of parity dataare generated for the virtual array, the presence of a degraded storagedevice, a storage configuration, a RAID level, and a number of storagedevices in the set.
 10. The apparatus of claim 1, wherein the buffer isa ping-pong buffer.
 11. The apparatus of claim 1, wherein: in a firstmode, data is received and written into the first portion of the bufferwhile data is read out of the second portion of the buffer; and in asecond mode, data is received and written into the second portion of thebuffer while data is read out of the first portion of the buffer. 12.The apparatus of claim 11, wherein the buffer is adapted to receive acontrol signal to select between the first mode and the second mode. 13.The apparatus of claim 1, wherein the buffer supports concurrent accessvia an input port of one portion of the buffer and an output port of theother portion of the buffer.
 14. The apparatus of claim 1, wherein thestate machine is implemented using logic gates.
 15. A method for readingfrom a physical storage-device array comprising a plurality of storagedevices using a buffer having a first portion and a second portion,wherein: the physical storage-device array has a plurality of sectorlevels, each sector level containing one sector on each storage devicein the array at corresponding addresses across the storage devices, andthe method comprises: (a) accessing first and second profiles, eachprofile defining (i) a virtual array associated with a selected set ofthe storage devices and (ii) one or more parameters used for accessinginformation from the virtual array; and (b) generating two or moresuccessive pairs of instructions, wherein: the first instruction of eachpair of instructions (i) is based on one or more parameters of a firstprofile and (ii) is an instruction to read, into the first portion ofthe buffer, one sector of data from each storage device of a firstvirtual array; and the second instruction of each pair of instructions(i) is based on one or more parameters of a second profile and (ii) isan instruction to read, into the second portion of the buffer, onesector of data from each storage device of a second virtual array. 16.The method of claim 15, wherein: in the first instruction, data fromeach sector of each storage device of the first virtual array is read ata first sector level; and in the second instruction, data from eachsector of each storage device of the second virtual array is read at asecond sector level.
 17. The method of claim 16, wherein, for successivepairs of instructions, the first and second instructions are repeated atrespective successive sector levels of the first and second virtualarrays.
 18. The method of claim 15, wherein the first and secondprofiles are associated with the same set of one or more storagedevices.
 19. The method of claim 18, wherein the first and secondinstructions are for reading from the same physical storage devices. 20.The method of claim 19, wherein the first and second instructions arefor reading from the same object.
 21. The method of claim 15, wherein:one of the storage devices is a degraded storage device; and the firstportion of the buffer is used for reconstructing information previouslystored on the degraded storage device.
 22. The method of claim 21,wherein the second portion of the buffer is used for streaming a mediaobject containing at least one of audio and video data.
 23. The methodof claim 15, wherein the one or parameters indicate one or more of: theamount of data read from or written to the storage devices at a time,whether chunks of parity data are generated for the virtual array, thepresence of a degraded storage device, a storage configuration, a RAIDlevel, and a number of storage devices in the set.
 24. The method ofclaim 15, wherein the buffer is a ping-pong buffer.
 25. The method ofclaim 15, wherein: in a first mode, data is received and written intothe first portion of the buffer while data is read out of the secondportion of the buffer; and in a second mode, data is received andwritten into the second portion of the buffer while data is read out ofthe first portion of the buffer.
 26. The method of claim 25, wherein thebuffer is adapted to receive a control signal to select between thefirst mode and the second mode.
 27. The method of claim 15, wherein thebuffer supports concurrent access via an input port of one portion ofthe buffer and an output port of the other portion of the buffer. 28.The method of claim 15, wherein the first and second instructions ofeach pair are generated using a state machine.
 29. The method of claim28, wherein the state machine is implemented using logic gates.